# Endangered and Threatened Wildlife and Plants; Threatened Species Status With Section 4(d) Rule for Monarch Butterfly and Designation of Critical Habitat

> Briefs, arguments, decisions, and more.

URL: https://www.frixlaw.com/law-library/documents/fr%3A2024-28855

## Record

- **Collection:** Federal Register
- **Document type:** Proposed Rule
- **Published:** December 12, 2024
- **Citation:** 89 FR 100662

## Text

DEPARTMENT OF THE INTERIOR
Fish and Wildlife Service
50 CFR Part 17
[Docket No. FWS-R3-ES-2024-0137; FXES1111090FEDR-256-FF09E21000]
RIN 1018-BE30
Endangered and Threatened Wildlife and Plants; Threatened Species Status With Section 4(d) Rule for Monarch Butterfly and Designation of Critical Habitat

AGENCY:

Fish and Wildlife Service, Interior.

ACTION:

Proposed rule.

SUMMARY:

We, the U.S. Fish and Wildlife Service (Service), propose to list the monarch butterfly (
Danaus plexippus
), the iconic orange and black butterfly, as a threatened species and designate critical habitat under the Endangered Species Act of 1973, as amended (Act). We propose to list the monarch butterfly as a threatened species with protective regulations under section 4(d) of the Act (a “4(d) rule”). Finalizing this rule as proposed would add this species to the List of Endangered and Threatened Wildlife and extend the Act's protections to the species. We also propose to designate critical habitat for the monarch butterfly under the Act. In total, approximately 4,395 acres (1,778 hectares) in Alameda, Marin, Monterey, San Luis Obispo, Santa Barbara, Santa Cruz, and Ventura Counties, California, fall within the boundaries of the proposed critical habitat designation. We also announce the availability of an economic analysis of the proposed designation of critical habitat for the monarch butterfly. We also are notifying the public that we have scheduled two informational meetings followed by public hearings on the proposed rule.

DATES:

We will accept comments received or postmarked on or before March 12, 2025. Comments submitted electronically using the Federal eRulemaking Portal (see
ADDRESSES
, below) must be received by 11:59 p.m. eastern time on the closing date.

Public informational meetings and public hearings:
We will hold two public informational meetings followed by public hearings. The first meeting and hearing will be from 6 p.m. to 8:30 p.m., eastern time, on January 14, 2025. To accommodate those in western time zones, the second meeting and hearing will be from 8 p.m. to 10:30 p.m., eastern time, on January 15, 2025.

ADDRESSES:

You may submit comments by one of the following methods:

(1)
Electronically:
Go to the Federal eRulemaking Portal:
https://www.regulations.gov.
In the Search box, enter FWS-R3-ES-2024-0137, which is the docket number for this rulemaking. Then, click on the Search button. On the resulting page, in the panel on the left side of the screen, under the Document Type heading, check the Proposed Rule box to locate this document. You may submit a comment by clicking on “Comment.”

(2)
By hard copy:
Submit by U.S. mail to: Public Comments Processing, Attn: FWS-R3-ES-2024-0137, U.S. Fish and Wildlife Service, MS: PRB/3W, 5275 Leesburg Pike, Falls Church, VA 22041-3803.

We request that you send comments only by the methods described above. We will post all comments on
https://www.regulations.gov.
This generally means that we will post any personal information you provide us (see Information Requested, below, for more information).

Availability of supporting materials:
Supporting materials, such as the species status assessment report, are available at
https://www.regulations.gov
at Docket No. FWS-R3-ES-2024-0137. If we finalize the critical habitat designation, we will make the coordinates or plot points or both from which the maps are generated available at
https://www.regulations.gov
at Docket No. FWS-R3-ES-2024-0137.

Public informational meeting and public hearing:
The public informational meeting and the public hearing will be held virtually using the Zoom platform. See Public Hearing, below, for more information.

FOR FURTHER INFORMATION CONTACT:

Barbara Hosler, Regional Listing Coordinator, U.S. Fish and Wildlife Service, Midwest Region Headquarters, 5600 American Blvd., Bloomington, MN 55437, telephone 517-580-0254, email:
monarch@fws.gov.
Individuals in the United States who are deaf, deafblind, hard of hearing, or have a speech disability may dial 711 (TTY, TDD, or TeleBraille) to access telecommunications relay services. Individuals outside the United States should use the relay services offered within their country to make international calls to the point-of-contact in the United States. Please see Docket No. FWS-R3-ES-2024-0137 on
https://www.regulations.gov
for a document that summarizes this proposed rule.

SUPPLEMENTARY INFORMATION:

Executive Summary

Why we need to publish a rule.
Under the Act, a species warrants listing if it meets the definition of an endangered species (in danger of extinction throughout all or a significant portion of its range) or a threatened species (likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range). If we determine that a species warrants listing, we must list the species promptly and designate the species' critical habitat to the maximum extent prudent and determinable. We have determined that the monarch butterfly meets the Act's definition of a threatened species; therefore, we are proposing to list it as such and proposing a designation of its critical habitat. Both listing a species as an endangered or threatened species and making a critical habitat designation can be completed only by issuing a rule through the Administrative Procedure Act rulemaking process (5 U.S.C. 551
et seq.
).

What this document does.
We propose to add the monarch butterfly as a threatened species to the List of Endangered and Threatened Wildlife with protective regulations under section 4(d) of the Act, and we propose the designation of critical habitat for the species.

The basis for our action.
Under the Act, we may determine that a species is an endangered or threatened species because of any of five factors: (A) The present or threatened destruction, modification, or curtailment of its habitat or range; (B) overutilization for commercial, recreational, scientific, or educational purposes; (C) disease or predation; (D) the inadequacy of existing regulatory mechanisms; or (E) other natural or manmade factors affecting its continued existence. We have determined that the monarch butterfly meets the definition of a threatened species due to the following threats: the ongoing impacts from loss and degradation of breeding, migratory, and overwintering habitat (from past conversion of grasslands and shrublands to agriculture and widespread use of herbicides; logging/thinning at overwintering sites in Mexico; urban development, senescence (
i.e.,
deterioration with age), and incompatible management of overwintering sites in California; and drought) (Factor A); exposure to insecticides (Factor E); and effects of climate change (Factor E).

To improve future conditions so that the monarch migratory populations stabilize and grow, we need to (1)

achieve a significant increase in the availability of milkweed and nectar plants in monarch breeding and migratory areas; (2) protect and enhance overwintering habitat; (3) avoid and minimize impacts to monarchs and their habitat from insecticides and herbicides; and (4) maintain public support for the conservation of monarch butterflies. Because of the monarch butterfly's general habitat use and wide distribution, all sectors of society, including the general public, have an opportunity to participate in a broad range of conservation efforts throughout the species' range.

Section 4(a)(3) of the Act requires the Secretary of the Interior (Secretary), to the maximum extent prudent and determinable, concurrently with listing designate critical habitat for the species. Section 3(5)(A) of the Act defines critical habitat as (i) the specific areas within the geographical area occupied by the species, at the time it is listed, on which are found those physical or biological features (I) essential to the conservation of the species and (II) which may require special management considerations or protections; and (ii) specific areas outside the geographical area occupied by the species at the time it is listed, upon a determination by the Secretary that such areas are essential for the conservation of the species. Section 4(b)(2) of the Act states that the Secretary must make the designation on the basis of the best scientific data available and after taking into consideration the economic impact, the impact on national security, and any other relevant impacts of specifying any particular area as critical habitat.

Information Requested

We intend that any final action resulting from this proposed rule will be based on the best scientific and commercial data available and be as accurate and as effective as possible. Therefore, we request comments or information from other governmental agencies, Native American Tribes, the scientific community, industry, or any other interested parties concerning this proposed rule. We particularly seek comments concerning:

(1) The species' biology, range, and population trends, including:

(a) Biological or ecological requirements of the species, including habitat requirements for feeding, breeding, and sheltering;

(b) Genetics and taxonomy;

(c) Historical and current range, including distribution patterns, alternative migratory pathways, and the locations of any additional populations of this species;

(d) Population dynamics and contributions from the nonmigratory populations, specifically resident monarchs in southern Florida, the Gulf Coast, the southern Atlantic Coast, and the southern Pacific Coast;

(e) Historical and current population levels, and current and projected trends; and

(f) Past and ongoing conservation measures for the species, its habitat, or both.

(2) Threats and conservation actions affecting the species, including:

(a) Factors that may be affecting the continued existence of the species, which may include habitat modification or destruction, overutilization, disease, predation, the inadequacy of existing regulatory mechanisms, or other natural or manmade factors;

(b) Biological, commercial trade, or other relevant data concerning any threats (or lack thereof) to this species;

(c) Existing regulations or conservation actions that may be addressing threats to this species; and

(d) Other potential threats that were not identified as key threats to the species.

(3) Additional information concerning the historical and current status of this species.

(4) Information to assist us with applying or issuing protective regulations under section 4(d) of the Act that may be necessary and advisable to provide for the conservation of the monarch butterfly. In particular, we seek information concerning:

(a) The extent to which we should include any of the Act's section 9 prohibitions in the 4(d) rule;

(b) Whether we should consider any modifications or additional exceptions from the prohibitions in the 4(d) rule;

(c) Whether the provisions related to the maintenance, enhancement, removal, or establishment of milkweed should be revised to include spatial or temporal restrictions or deferments;

(d) Whether we should include an exception for the use of pesticides and, if so, what measures are reasonable, feasible, and adequate to reduce or offset pesticide exposure to monarchs from agricultural and non-agricultural uses (
e.g.,
rangeland, rights-of-way, forestry, commercial areas, and mosquito control), including measures for specific classes of pesticides (
e.g.,
herbicides, insecticides), pesticide uses, and application methods;

(e) Whether we should include an exception for direct impacts from transportation and energy infrastructure, including mortality from collisions with wind turbines; and

(f) Whether we could improve or modify our approach to the 4(d) rule in any way to provide for greater public participation and understanding, or to better accommodate public concerns and comments.

(5) Specific information related to critical habitat, such as the following:

(a) The amount and distribution of monarch butterfly habitat;

(b) Any additional areas occurring within the range of the species in the United States, especially in breeding, migratory, or overwintering areas, that should be included in the critical habitat designation because they (i) are occupied at the time of listing and contain the physical or biological features that are essential to the conservation of the species and that may require special management considerations, or (ii) are unoccupied at the time of listing and are essential for the conservation of the species; and

(c) Special management considerations or protection that may be needed in critical habitat areas we are proposing, including managing for the potential effects of climate change.

(6) Land use designations and current or planned activities in the subject areas and their possible impacts on proposed critical habitat.

(7) Any probable economic, national security, or other relevant impacts of designating any area that may be included in the final designation, and the related benefits of including or excluding specific areas.

(8) Information on the extent to which the description of probable economic impacts in the economic analysis is a reasonable estimate of the likely economic impacts and any additional information regarding probable economic impacts that we should consider.

(9) Whether any specific areas we are proposing for critical habitat designation should be considered for exclusion under section 4(b)(2) of the Act, and whether the benefits of potentially excluding any specific area outweigh the benefits of including that area. If you think we should exclude any additional areas, please provide information supporting a benefit of exclusion.

(10) Whether we could improve or modify our approach to designating critical habitat in any way to provide for greater public participation and understanding, or to better accommodate public concerns and comments.

Please include sufficient information with your submission (such as scientific journal articles or other publications) to

allow us to verify any scientific or commercial information you include.

Please note that submissions merely stating support for, or opposition to, the action under consideration without providing supporting information, although noted, do not provide substantial information necessary to support a determination. Section 4(b)(1)(A) of the Act directs that determinations as to whether any species is an endangered or a threatened species must be made solely on the basis of the best scientific and commercial data available, and section 4(b)(2) of the Act directs that the Secretary shall designate critical habitat on the basis of the best scientific data available.

You may submit your comments and materials concerning this proposed rule by one of the methods listed in
ADDRESSES
. We request that you send comments only by the methods described in
ADDRESSES
.

If you submit information via
https://www.regulations.gov,
your entire submission—including any personal identifying information—will be posted on the website. If your submission is made via a hardcopy that includes personal identifying information, you may request at the top of your document that we withhold this information from public review. However, we cannot guarantee that we will be able to do so. We will post all hardcopy submissions on
https://www.regulations.gov.

Comments and materials we receive, as well as supporting documentation we used in preparing this proposed rule, will be available for public inspection on
https://www.regulations.gov.

Our final determination may differ from this proposal because we will consider all comments we receive during the comment period as well as any information that may become available after this proposal. Based on the new information we receive (and, if relevant, any comments on that new information), we may conclude that the species is endangered instead of threatened, or we may conclude that the species does not warrant listing as either an endangered species or a threatened species. For critical habitat, our final designation may not include all areas proposed, may include some additional areas that meet the definition of critical habitat, or may exclude some areas if we find the benefits of exclusion outweigh the benefits of inclusion and exclusion will not result in the extinction of the species. In addition, we may change the parameters of the prohibitions or the exceptions to those prohibitions in the protective regulations issued or applied under section 4(d) of the Act if we conclude it is appropriate in light of comments and new information received. For example, we may expand the prohibitions if we conclude that the protective regulation as a whole, including those additional prohibitions, is necessary and advisable to provide for the conservation of the species. Conversely, we may establish additional or different exceptions to the prohibitions in the final rule if we conclude that the activities would facilitate or are compatible with the conservation and recovery of the species. In our final rule, we will clearly explain our rationale and the basis for our final decision, including why we made changes, if any, that differ from this proposal.

Public Hearing

We have scheduled two public informational meetings with public hearings on this proposed rule for the monarch butterfly. We will hold the public informational meetings and public hearings on the dates and times listed above under
Public informational meeting and public hearing
in
DATES
. We are holding the public informational meetings and public hearings via the Zoom online video platform and via teleconference so participants can attend remotely. For security purposes, registration is required. To listen and view a meeting and hearing via Zoom, listen to a meeting and hearing by telephone, or provide oral public comments at a public hearing by Zoom or telephone, you must register. For information on how to register, or if you encounter problems joining Zoom the day of the meeting, visit
https://www.fws.gov/species/monarch-danaus-plexippus.
Registrants will receive the Zoom link and the telephone number for the public informational meeting and public hearing for which they have registered. If applicable, interested members of the public not familiar with the Zoom platform should view the Zoom video tutorials (
https://support.zoom.us/hc/en-us/articles/206618765-Zoom-video-tutorials
) prior to the public informational meetings and public hearings.

The public hearings will provide interested parties an opportunity to present verbal testimony (formal, oral comments) regarding this proposed rule. While the public informational meetings will be opportunities for dialogue with the Service, the public hearings are not: They are forums for accepting formal verbal testimony. In the event there is a large attendance, the time allotted for oral statements may be limited. Therefore, anyone wishing to make an oral statement at a public hearing for the record is encouraged to provide a prepared written copy of their statement to us through the Federal eRulemaking Portal, or U.S. mail (see
ADDRESSES
, above). There are no limits on the length of written comments submitted to us. Anyone wishing to make an oral statement at a public hearing must register before the hearing
https://www.fws.gov/species/monarch-danaus-plexippus.
The use of a virtual public hearing is consistent with our regulations at 50 CFR 424.16(c)(3).

Previous Federal Actions

On August 26, 2014, we received a petition from the Center for Biological Diversity, Center for Food Safety, Xerces Society for Invertebrate Conservation, and Dr. Lincoln Brower, requesting that we list the monarch butterfly as a threatened species under the Act. On December 31, 2014, we published a 90-day finding that the petition presented substantial scientific or commercial information, indicating that listing the monarch butterfly may be warranted (79 FR 78775). On December 17, 2020, we published a 12-month finding that listing the species as an endangered or threatened species is warranted but precluded by higher priority actions (85 FR 81813). The species remained so designated in the annual candidate notices of review on May 3, 2022 (87 FR 26152), and June 27, 2023 (88 FR 41560).

Peer Review

A species status assessment (SSA) team prepared an SSA report for the monarch butterfly. The SSA team was composed of Service biologists, in consultation with other species experts. The SSA report represents a compilation of the best scientific and commercial data available concerning the status of the species, including the impacts of past, present, and future factors (both negative and beneficial) affecting the species.

In accordance with our joint policy on peer review published in the
Federal Register
on July 1, 1994 (59 FR 34270), and our August 22, 2016, memorandum updating and clarifying the role of peer review in listing actions under the Act (
https://www.fws.gov/sites/default/files/documents/peer-review-policy-directors-memo-2016-08-22.pdf
), we solicited independent scientific review of the information contained in the monarch butterfly SSA report. In 2018, we sent the first version (1.0) of the SSA report to 6 independent peer reviewers and received 5 responses. In 2020, we sent the second version (2.0) of the SSA report to 3 peer reviewers and received 2 responses. In 2023, we sent the SSA

report version 2.2 to 13 peer reviewers and received 7 responses. No individual peer reviewed the SSA report more than once. Results of this structured peer review process can be found at
https://www.regulations.gov.
In preparing this proposed rule, we incorporated the results of these reviews, as appropriate, into the SSA report, which is the foundation for this proposed rule.

Summary of Peer Reviewer Comments

As discussed in Peer Review above, we received comments from 14 total peer reviewers on the draft SSA report. We reviewed all comments we received from the peer reviewers for substantive issues and new information regarding the contents of the SSA report. The peer reviewers provided additional references, clarifications, and suggestions.

We updated the SSA report based on these peer reviewers' comments, including restructuring several sections within the report, revising our description of adaptive capacity, clarifying specific points and terminology where appropriate, and adding additional details and suggested references when needed. Additional peer reviewer comments are addressed in the following summary, as well as throughout the Summary of Biological Status and Threats, below, and were incorporated into the SSA report as appropriate (Service 2024a, entire).

Comment 1:
Two reviewers asked us to elaborate on whether our assessment of resilience or the extinction threshold for western migratory monarchs should change based on a few years of numbers that have bounced back above 200,000 after the low of less than 2,000 individuals in the winter of 2019-2020.

Our response:
It is plausible that migratory monarchs experienced good weather that resulted in abundant and well-timed milkweed and nectar resources across their breeding habitat in western North America in 2021, which provided for a significant increase in the overwintering count from below 2,000 individuals in 2019-2020. With only a few years of improved population numbers, we are currently unable to determine whether the western migratory population's status is improving or if these data support the continuation of its fluctuation and decline. While this does not change our current assessment of the population's resilience, we have added text clarifying this uncertainty to our SSA report to further address this comment.

We also revisited our extinction thresholds in western North America. In the model presented in our SSA, we use a range of extinction thresholds, which are informed by scientific literature (
i.e.,
Schultz et al. 2017, entire; Wells et al. 1990, p. 124). With the observed western population rebound, these estimates may be conservative. While the extinction thresholds we used are currently the best available, some uncertainty remains about the size of extinction thresholds and the time lag necessary to confirm assumptions about population stability or extinction trajectories. Therefore, we could be either overestimating or underestimating extinction risk depending on the accuracy of the thresholds.

Comment 2:
One reviewer questioned our reliance on overwintering counts to inform our understanding of eastern North American monarch butterfly population trends, in contrast to using surveys at other locations and at other times during the year.

Our response:
Monitoring at overwintering sites allows for the most practical and direct assessment of annual population levels, where the near-entirety of the migratory population can be evaluated consistently and comprehensively, reflecting the full range of threats and population dynamics encountered over the prior annual breeding and migration cycle. The question about the optimal approach for annual census of monarch butterfly populations (
e.g.,
overwintering data v. non-overwintering data) has been addressed in a more recent review of the methodology, which concluded that the overwintering area of the eastern North American population is an accurate representation of the size of the population (Pleasants et al. 2024, p. 62). Our analysis of this topic based on the preponderance of scientific evidence is incorporated into the most recent version of the SSA report (Service 2024a, pp. 70-76).

Comment 3:
Two reviewers noted that the decline of the eastern North American monarchs may have slowed or stabilized for the past decade or more, potentially due to an associated slowing or plateau of habitat loss. With this potential slowing or stabilization, one of these reviewers asked if the extinction risk may have thus decreased. They also noted differences in population decline based on the modeling assumption of density-dependence or independence.

Our response:
The estimates of population viability (and extinction risk) presented in the SSA report are based on the overall population trajectory (growth rate), along with the current population size and the fluctuations exhibited year-to-year. Growth rate and variability are derived using time series data, which may be influenced by the number of years being analyzed.

Different analyses have explored the possibility that past declines in milkweed, or habitat more generally, alongside the expansion of genetically modified crops and associated herbicides and pesticides, may have effectively plateaued in recent years. Some evidence has emerged of a change in trajectory for the eastern population since 2014, but as of 2020, statistical support for such a change was insufficient (Thogmartin et al. 2020, entire). More recently, an analysis of overwintering data for the eastern population estimated a median annual rate of growth of 0.93 (0.67-1.30, 95-percent confidence interval (CI)). These results indicate that the population is declining, even when assuming relatively stable land use and landcover change relative to the conditions that had driven steeper population declines in previous decades (Thogmartin 2024, entire).

Other published analyses estimate monarch population growth rates and persistence/extinction risks using alternative models and different data sets. These include non-overwintering population data, different time-series of annual population estimates, different modeling assumptions about density-dependence, and different relationships between population size and growth (
i.e.,
increased density of monarchs will result in decreased population growth and decreased density of monarchs will result in increased population growth). One recent analysis (Meehan and Crossley 2023, entire) used a variable change model to characterize the trend in eastern North American monarch abundance, suggesting that the monarch population has rebounded after a period of decline, thereby leading to a markedly lower risk of extinction relative to other assessments (
e.g.,
Semmens et al. 2016, entire; Schultz et al. 2017, entire; Thogmartin 2024, entire), including our SSA analyses. Meehan and Crossley (2023, entire) assume density dependence, which constrains the uncertainty associated with the predicted abundance estimates (small populations recover faster under density-dependent assumptions than density-independent assumptions). Though their density dependent model provides a good fit for the 10-year dataset they analyzed, the models cannot identify the mechanism behind the apparent decline in growth rate as populations increase. The assessment by Meehan and Crossley (2023, entire) did not incorporate the impacts of changing future conditions.

Another analysis that also did not incorporate the impacts of changing future conditions assumes the variability in the population numbers is now driven by environmental and demographic stochasticity (Thogmartin 2024, entire). This approach is more conservative because it results in increased variability at small populations sizes, as compared to the assumption of density dependence (which assumes increased per capita growth when populations are small). In this modeling approach, when population abundance has been reduced to a low level, demographic and environmental stochasticity alone (and not necessarily a declining trend) are now the driving factors, resulting in an increased risk of extinction. After careful examination, our estimates in the SSA for the annual rate of growth, population viability, and extinction risk continue to be in line with the best available information.

Comment 4:
One reviewer of a later version of the SSA asked for additional clarification on why the model for monarch butterfly extinction was not re-run with updated data that had been collected since the first version of the SSA.

Our response:
Based on our previous sensitivity testing of the model presented in the SSA, updated values would not significantly change the output and results, and thus would not change the outcomes in our report. Therefore, instead of rerunning the model, we instead prioritized evaluating new literature and information that might have changed the SSA analyses and conclusions. We provide further clarification and explanation in the updated SSA report (Service 2024a, p. 149).

Comment 5:
One reviewer commented that nonmigratory monarch butterflies might persist, even if the migratory monarchs were to become extirpated. The reviewer also suggested that one potential reason for the declines in migratory individuals in the West could be due to a shift to more individuals existing in nonmigratory populations during winter.

Our response:
According to recent research, the resident (nonmigratory) population in California is not sufficient to make up for the loss of the migratory population in western North America; there are still orders of magnitude fewer butterflies now than in the recent past (Crone and Schultz 2021, p. 1535). Also, the resident population probably lacks the demographic capacity to expand its range inland during summer months. Resident populations of monarch butterflies build up high levels of a protozoan parasite,
Ophryocystis elektroscirrha
(
OE
), at least in part due to the absence of migratory culling and migratory escape (Satterfield et al. 2015, pp. 4-5). In California, about 8 percent of migratory monarch butterflies are infected with
OE,
compared to about 75 percent of residents (Satterfield et al. 2016, p. 346). Similarly, for the eastern North American population, less than 10 percent of migratory monarch butterflies are infected with
OE,
compared to 75-100 percent of nonmigratory monarchs in Florida (Altizer and de Roode 2015, p. 91).
OE
-infected monarch butterflies experience lower survival, lower egg-laying rates, and produce about 0.8 adult daughters per female (Crone and Schultz 2021, Supplement S4). This rate of increase is enough for resident monarch butterfly populations to persist in urban areas, but it does not provide the ability to rapidly colonize the other Western States (Crone and Schultz 2021, p. 1536). We acknowledge alternative overwintering strategies, including nonmigratory monarchs, in the uncertainties section of the SSA report (Service 2024a, pp. 70-76).

I. Proposed Listing Determination

Background

A thorough review of the taxonomy, life history, and ecology of the monarch butterfly (
Danaus plexippus;
referred to as “monarch” or “monarch butterfly” herein) is presented in the SSA report version 2.3 (Service 2024a, entire).

The monarch is a brightly colored butterfly species, native to North America, with a range that has expanded west via human assistance to many islands in the Pacific Ocean and to the east to the Iberian Peninsula to now occupy 90 countries, islands, and island groups. Despite the expansion, over 90 percent of monarchs worldwide continue to live and migrate in their native range in North America. The species requires habitat with milkweed (genus
Asclepias
or closely related genera) as a larval host plant and floral nectar sources for adults. Migratory monarchs in North America also use overwintering habitat, where the adults cluster on trees.

Adult monarch butterflies are large and conspicuous, with bright-orange wings surrounded by a black border and covered with black veins. The black border has a double row of white spots, present on the upper side and lower side of forewings and hindwings (Bouseman and Sternburg 2001, p. 222). Adult monarchs are sexually dimorphic, with males having narrower wing venation and scent patches (Commission for Environmental Cooperation (CEC) 2008, p. 11). The monarch butterfly has bright and contrasting coloration, which serves as a warning to predators that eating them can be toxic.

The migratory North American populations are located east and west of the Rocky Mountains and overwinter primarily at sites with specific microhabitats in central Mexico and California, respectively. In central Mexico, the species primarily overwinters in mountainous regions where the monarchs form dense clusters mainly on oyamel fir trees (
Abies religiosa
). Western monarchs spend the fall and winter at tree groves along the California coast, northern Baja California, Mexico, and at a few inland sites in the Saline Valley of California. These groves are populated by a variety of tree species, including blue gum eucalyptus (
Eucalyptus globulus
), Monterey pine (
Pinus radiata
), Monterey cypress (
Cupressus macrocarpa
), and others. The overwintering sites for both the eastern and western North American migratory monarch populations provide protection from the elements and a microhabitat conducive for winter survival. In contrast, monarchs in habitats with suitable winter climates (
e.g.,
some areas in California and Florida) may breed year-round without migrating.

During the breeding season for monarchs, adults lay their eggs on milkweed, and larvae emerge after 2 to 5 days (Zalucki 1982, p. 242; CEC 2008, p. 12). Larvae develop through five larval instars (intervals between molts) over a period of 9 to 18 days, feeding on milkweed and sequestering toxic cardenolides as a defense against predators (Parsons 1965, p. 299). The larva then pupates into a chrysalis before eclosing 6 to 14 days later as an adult butterfly. Monarchs produce multiple generations during the breeding season, with most adult butterflies living approximately 2 to 5 weeks; however, migrating and overwintering adults enter into reproductive diapause (suspended reproduction) and live 6 to 9 months (Cockrell et al. 1993, pp. 245-246; Herman and Tatar 2001, p. 2509).

Regulatory and Analytical Framework

Regulatory Framework

Section 4 of the Act (16 U.S.C. 1533) and the implementing regulations in title 50 of the Code of Federal Regulations (CFR) set forth the procedures for determining whether a species is an endangered species or a threatened species, issuing protective

regulations for threatened species, and designating critical habitat for endangered and threatened species.

The Act defines an “endangered species” as a species that is in danger of extinction throughout all or a significant portion of its range, and a “threatened species” as a species that is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range. The Act requires that we determine whether any species is an endangered species or a threatened species because of any of the following factors:

(A) The present or threatened destruction, modification, or curtailment of its habitat or range;

(B) Overutilization for commercial, recreational, scientific, or educational purposes;

(C) Disease or predation;

(D) The inadequacy of existing regulatory mechanisms; or

(E) Other natural or manmade factors affecting its continued existence.

These factors represent broad categories of natural or human-caused actions or conditions that could have an effect on a species' continued existence. In evaluating these actions and conditions, we look for those that may have a negative effect on individuals of the species, as well as other actions or conditions that may ameliorate any negative effects or may have positive effects.

We use the term “threat” to refer in general to actions or conditions that are known to or are reasonably likely to negatively affect individuals of a species. The term “threat” includes actions or conditions that have a direct impact on individuals (direct impacts), as well as those that affect individuals through alteration of their habitat or required resources (stressors). The term “threat” may encompass—either together or separately—the source of the action or condition or the action or condition itself.

However, the mere identification of any threat(s) does not necessarily mean that the species meets the statutory definition of an “endangered species” or a “threatened species.” In determining whether a species meets either definition, we must evaluate all identified threats by considering the species' expected response and the effects of the threats—in light of those actions and conditions that will ameliorate the threats—on an individual, population, and species level. We evaluate each threat and its expected effects on the species, then analyze the cumulative effect of all of the threats on the species as a whole. We also consider the cumulative effect of the threats in light of those actions and conditions that will have positive effects on the species, such as any existing regulatory mechanisms or conservation efforts. The Secretary determines whether the species meets the definition of an “endangered species” or a “threatened species” only after conducting this cumulative analysis and describing the expected effect on the species.

The Act does not define the term “foreseeable future,” which appears in the statutory definition of “threatened species.” Our implementing regulations at 50 CFR 424.11(d) set forth a framework for evaluating the foreseeable future on a case-by-case basis, which is further described in the 2009 Memorandum Opinion on the foreseeable future from the Department of the Interior, Office of the Solicitor (M-37021, January 16, 2009; “M-Opinion,” available online at
https://www.doi.gov/sites/doi.opengov.ibmcloud.com/files/uploads/M-37021.pdf
). The foreseeable future extends as far into the future as the U.S. Fish and Wildlife Service and National Marine Fisheries Service (hereafter, the Services) can make reasonably reliable predictions about the threats to the species and the species' responses to those threats. We need not identify the foreseeable future in terms of a specific period of time. We will describe the foreseeable future on a case-by-case basis, using the best available data and taking into account considerations such as the species' life-history characteristics, threat projection timeframes, and environmental variability. In other words, the foreseeable future is the period of time over which we can make reasonably reliable predictions. “Reliable” does not mean “certain”; it means sufficient to provide a reasonable degree of confidence in the prediction, in light of the conservation purposes of the Act.

Analytical Framework

The SSA report documents the results of our comprehensive biological review of the best scientific and commercial data regarding the status of the species, including an assessment of the potential threats to the species. The SSA report does not represent our decision on whether the species should be proposed for listing as an endangered or threatened species under the Act. However, it does provide the scientific basis that informs our regulatory decisions, which involve the further application of standards within the Act and its implementing regulations and policies.

To assess monarch butterfly viability, we used the three conservation biology principles of resiliency, redundancy, and representation (Shaffer and Stein 2000, pp. 306-310). Briefly, resiliency is the ability of the species to withstand environmental and demographic stochasticity (for example, wet or dry, warm or cold years); redundancy is the ability of the species to withstand catastrophic events (for example, droughts, large pollution events); and representation is the ability of the species to adapt to both near-term and long-term changes in its physical and biological environment (for example, climate conditions, pathogens). In general, species viability will increase with increases in resiliency, redundancy, and representation (Smith et al. 2018, p. 306). Using these principles, we identified the species' ecological requirements for survival and reproduction at the individual, population, and species levels, and described the beneficial and risk factors influencing the species' viability.

The SSA process can be categorized into three sequential stages. During the first stage, we evaluated the individual species' life-history needs. The next stage involved an assessment of the historical and current condition of the species' demographics and habitat characteristics, including an explanation of how the species arrived at its current condition. The final stage of the SSA involved making predictions about the species' responses to positive and negative environmental and anthropogenic influences. Throughout all of these stages, we used the best available information to characterize viability as the ability of a species to sustain populations in the wild over time, which we then used to inform our regulatory decision.

The following is a summary of the key results and conclusions from the SSA report; the full SSA report can be found at Docket FWS-R3-ES-2024-0137 on
https://www.regulations.gov.

Summary of Biological Status and Threats

In this discussion, we review the biological condition of the species and its resources, and the threats that influence the species' current and future condition, in order to assess the species' overall viability and the risks to that viability.

Species Needs

Individual-Level Ecology

The monarch life cycle varies by geographic location. Migratory monarchs in North America are the

ancestral population for all other monarch populations (Pierce et al. 2014, p. 4; Zhan et al. 2014, p. 318; Hemstrom et al. 2022, pp. 4551-4552), which dispersed from North America via human assistance, potentially aided through wind dispersal events (Brower 1995, p. 354), and now occur in Central and South America; Australia; New Zealand; islands of the Pacific and Caribbean, and elsewhere (Malcolm and Zalucki 1993, pp. 3-5) where milkweed (their larval host plant) was already present or introduced. In a few parts of North America (such as parts of Florida, the Gulf Coast, and California) and in most areas outside North America, monarchs breed year-round, repeatedly following the above-referenced life cycle throughout the year. However, monarchs in temperate climates such as eastern and western North America undergo long-distance migration, where the migratory generation of adults is in reproductive diapause and lives for an extended period of time (Herman and Tatar 2001, p. 2509).

In the fall, in both eastern and western North America, monarchs begin migrating to their respective overwintering sites. This migration can take monarchs distances of over 2,000 miles (mi) (3,000 kilometers (km)) (Urquhart and Urquhart 1978, p. 1760) and last for over 2 months (Brower 1996, p. 93). The vast majority of migratory individuals in eastern North America fly south or southwest to mountainous overwintering grounds in central Mexico, although some minor alternative migratory routes have also been suggested (Dockx et al. 2023, p. 314). Migratory individuals in western North America generally fly shorter distances south and west to overwintering groves along the California coast into northern Baja California (Solensky 2004, p. 79). Data from monarchs tagged in the southwestern States in the fall suggest that those in Nevada migrate to California, those in New Mexico migrate to Mexico, and those in Arizona migrate to either Mexico or California (Southwest Monarch Study Inc. 2018, unpaginated). In early spring (February-March), surviving monarchs break diapause and mate at the overwintering sites before dispersing (Leong et al. 1995, p. 46; van Hook 1996, pp. 16-17). The same individuals that undertook the initial southward migration begin flying back through the breeding grounds, and their offspring start the cycle of generational migration over again (Malcolm et al. 1993, p. 262).

The spring monarch migrations in eastern and western North America represent massive annual range expansions. In eastern North America, monarchs travel north in the spring, from Mexico to Canada, over two to three successive generations, breeding along the way (Flockhart et al. 2013, pp. 4-5). Individual monarchs may disperse as far north as they can physiologically tolerate based on climatic conditions and available vegetation; the most specific predictors of the northern distribution of individual monarchs are monthly mean temperature and precipitation (Flockhart et al. 2013, p. 4; Flockhart et al. 2017, p. 2570). The number of generations of monarchs produced in a given year in a migratory population can vary between three and five and is dependent upon environmental conditions (Brower 1996, p. 100). While a majority of the eastern monarchs shift to the more northern reaches of their range, western monarchs continue to occupy and breed in warmer climates throughout the summer, while also expanding to include the farther reaches of their range. In the spring in western North America, monarchs migrate north and east over multiple generations from coastal California toward the Rockies and to the Pacific Northwest (Urquhart and Urquhart 1977, p. 1585; Nagano et al. 1993, entire). In the southwestern States, migrating monarchs tend to occur more frequently near water sources such as rivers, creeks, roadside ditches, and irrigated gardens (Morris et al. 2015, p. 100).

In addition to the monarchs that overwinter by clustering at known overwintering roost sites, some monarchs in North America also breed year-round or breed throughout the winter. These individuals require the breeding habitat and suitable climate (table 1) and are discussed further in Species-Level Ecology, below.

To facilitate the massive annual spring range expansion, adult monarch butterflies require a diversity of blooming nectar resources, which they feed on both throughout their migration routes and in their breeding grounds (spring through fall). Monarchs also need milkweed (for both oviposition and larval feeding) embedded within this diverse nectaring habitat. The correct phenology, or timing, of both monarchs and nectar plants and milkweed is important for monarch survival. The position of these resources on the landscape is important as well (see Population-Level Ecology, below). In western North America, nectar and milkweed resources are often associated with riparian corridors, and milkweed may function as the principal nectar source for monarchs in more arid regions (Dingle et al. 2005, p. 494; Pelton et al. 2018, p. 18; Waterbury and Potter 2018, p. 38; Dilts et al. 2018, p. 8). Individuals need nectar and milkweed resources year-round in nonmigratory populations. Additionally, many monarchs use a variety of roosting trees along the fall migration route (table 1).

Migratory individuals of eastern and western North America require a specific microclimate at overwintering sites. The eastern population of monarchs overwinter in Mexico, where this microclimate is provided by forests primarily composed of oyamel fir trees, on which the monarchs form dense clusters (Williams and Brower 2015, pp. 109-110). These cluster formations provide a buffer for individuals against freezing temperatures, lipid depletion, and desiccation (Brower et al. 2008, p. 186). The sites used for overwintering primarily occur in mountainous areas west of Mexico City located between elevations of 9,500 feet (ft) (2,900 meters (m)) and 10,800 ft (3,300 m) (Slayback and Brower 2007, p. 147). The temperature must remain cool enough to prevent excessive lipid depletion (Alonso-Mejía et al. 1997, p. 935), while at the same time staying warm enough to prevent freezing (Anderson and Brower 1996, pp. 111-113). Exposure to these cooler temperatures also helps orient the monarchs northward in the spring (Guerra and Reppert 2013, pp. 421-422). The oyamel fir forest provides essential protection from the elements, including rain, snow, wind, hail, and excessive solar radiation (Williams and Brower 2015, p. 109). Many sites also provide a source of hydration via nectar plants or a water source (Brower et al. 1977, pp. 237-238). Most of the observed overwintering sites are located within the Monarch Butterfly Biosphere Reserve, which covers more than 138,000 acres (ac) (56,000 hectares (ha)) (Vidal and Rendón-Salinas 2014, p. 169; Ramírez et al. 2015, p. 158).

Migratory monarchs in the western population primarily overwinter in groves along the coast of California and Baja California (Jepsen and Black 2015, p. 149). The location and structure of these sites provide the specific microclimate (although different from the Mexico overwintering microclimate) needed for survival in the western overwintering areas. Approximately 400 groves have been known to be occupied, but only a portion of these sites is occupied in any given year. These sites, typically close to the coast, span approximately 761 mi (1,225 km) of coastline (COSEWIC 2010, p. 10). These groves are populated by a variety of tree species, including blue gum eucalyptus, Monterey pine, and Monterey cypress

(Griffiths and Villablanca 2015, pp. 41, 46-47), all of which act as roost trees. These groves provide indirect sunlight for the overwintering monarchs, sources of moisture for hydration, defense against freezing temperatures, and protection against strong winds (Tuskes and Brower 1978, p. 149; Leong 1990, pp. 908-910, Leong 1999, p. 213). The close proximity to the coast (average distance of 1.47 mi (2.37 km)) also provides a mild winter climate (Leong et al. 2004, p. 180).

Table 1—Individual-Level Requisites for Monarch Survival and Reproduction

Life stage
Requirements
Description

Egg, larva, and adult—breeding
Milkweed resources
Healthy and abundant milkweed is needed for oviposition and larval consumption.

Adult—breeding and migration
Nectar resources
Sufficient quality and quantity of nectar from flowers is needed for adult feeding throughout the breeding and migration seasons.

Adult—overwintering
Suitable habitat for overwintering

Habitat that provides a specific roosting microclimate for overwintering: protection from the elements (
e.g.,
rain, wind, hail, excessive radiation) and moderate temperatures that are warm enough to prevent freezing yet cool enough to prevent lipid depletion. Nectar and clean water sources located near roosting sites.

Adult—migration
Connectivity and phenology
Nectar and milkweed resources along the migration route when butterflies are present; the size and spatial arrangement of habitat patches are generally thought to be important aspects, but currently unknown. Roosting sites may also be important for monarchs along their fall migration route.

Population-Level Ecology

Monarchs, like many insects, are sensitive to environmental conditions (temperature and precipitation) and can experience large swings in population numbers from year to year in response to these conditions (Rendón-Salinas et al. 2015, p. 3; Schultz et al. 2017, pp. 345-346). During favorable conditions, monarch survival and reproductive rates are high and population numbers increase; conversely, when environmental conditions are unfavorable, survival and reproductive rates are low and population numbers can plummet. Thus, to successfully recruit over generations and years, they must be capable of withstanding large swings in population sizes (
N
). Specifically, they need a positive population growth rate (lambda, or λ) that is sufficient for the species to recover from annual stochasticity.

To support a strong growth rate, monarch populations require sufficiently large population sizes and an adequate quantity and quality of habitat to support these large population sizes. Large population sizes also help maintain genetic health and facilitate thermoregulation during the winter, which is important for good physical health. A sufficiently large population size may also be important for finding mates and repelling predators (Malcolm 2018, pers. comm.). Both migratory and breeding habitat need to be distributed throughout the landscape to ensure connectivity, allowing monarchs within a population to reach all portions of their range and to maximize lifetime fecundity (Zalucki and Lammers 2010, p. 84; Miller et al. 2012, p. 2).

Species-Level Ecology

The ecological requisites at the species level include having a sufficient number and distribution of healthy populations to ensure the species can withstand annual variation in its environment (resiliency), catastrophes (redundancy), and novel biological and physical changes in its environment (representation). We describe the monarch's requirements for resiliency, redundancy, and representation below.

Resiliency

Monarch populations with a robust growth rate, large effective population size, and suitable quality, quantity, and distribution of habitat conditions are better able to withstand and recover from environmental variability and stochastic perturbations (
e.g.,
storms, dry years) than those populations that are less demographically, genetically, or physically healthy. Given the monarch's sensitivity to environmental conditions, which can cause large swings in population numbers year-to-year (Rendón-Salinas et al. 2015, p. 3), monarchs occupying a diversity of environmental conditions and being widely distributed helps guard against populations being exposed to adverse conditions concurrently, and thus, fluctuating in synchrony. Asynchronous dynamics within and among populations minimizes the chances of concurrent losses, and thus, provides species resiliency.

Redundancy

Monarch redundancy is best achieved by having a broad geographic distribution of monarchs relative to the spatial occurrence of catastrophic events. For the eastern North American migratory population, potential catastrophic events include extreme storms when monarchs are densely congregated and widespread drought. For the western North American migratory population, potential catastrophic events include widespread drought and co-occurrence of poor environmental conditions and low population abundance. For dispersed non-migratory populations, potentially catastrophic events include sea level rise, which can inundate habitat, and lethal high temperatures.

Migratory North American monarchs are distributed across vast and diverse habitats across much of North America during their breeding and migratory seasons. However, while overwintering, most migratory monarchs are highly concentrated at sites in Mexico and California. Nonmigratory monarchs in North America are distributed in warmer, suitable areas throughout the year, including in parts of Florida, the Gulf Coast, and California. A very small proportion of the species is also distributed in nonnative or naturalized populations across diverse habitats throughout 90 countries, islands, and island groups.

Representation

The monarch's ability to withstand novel changes in its environment is influenced by its adaptive capacity, which is primarily a function of the species' breadth of variation in biological traits and genetic diversity. Without such variation, species are less responsive to change and more prone to extinction (Spielman et al. 2004, p. 15263). Additionally, as populations with higher genetic diversity can more quickly adapt to novel changes, species with genetically healthy populations are better able to adapt (Ofori et al. 2017, p.

2). Below we describe monarch adaptive capacity by using the best available data.

Migratory monarchs in North America are the ancestral population for all other monarch populations around the world (Pierce et al. 2014, p. 4; Zhan et al. 2014, p. 318; Hemstrom et al. 2022, pp. 4551-4552). Their unique genetics separate them from nonmigratory monarchs (Freedman et al. 2021, p. 7). Genetic sampling indicates that the monarchs from eastern and western North America have continued interchange between the two populations contributing to low genetic differentiation (Talla et al. 2020, p. 2573; Freedman et al. 2021, pp. 7-8). During the annual breeding season, the North American migratory populations use a vast and diverse array of habitats. Following a long-distance migration through more varied habitats and conditions, the North American migratory monarchs use sites with a diversity of physical structures and climatic conditions for overwintering. Having monarchs occupying areas of unique ecological diversity guards against losses of adaptive capacity due to stochastic or catastrophic events. Nonmigratory monarchs represent a life-history strategy that exposes them to selection pressures that differ from migratory monarchs.

Eastern North American monarchs undergo long-distance migration every fall, a behavior that differentiates this population from nonmigratory populations or from migratory populations that fly shorter distances and to different locations. The migratory phenotype of monarchs in the eastern migratory population is distinct from monarchs in other populations that may have latent migratory phenotypes (Tenger-Trolander et al. 2019, p. 14673). This migratory phenotype consists of both reproductive diapause and directional flight orientation to the south, and this migratory behavior of monarchs is remarkably sensitive to genetic and environmental change (Tenger-Trolander et al. 2019, p. 14673). Monarchs from the eastern North American migratory population tend to have larger bodies and larger elongated wings compared to monarchs from most nonmigratory populations (Altizer and Davis 2010, pp. 1023-1025). Eastern North American migratory monarchs inhabit 70 percent or more of the total area occupied by monarch butterflies in North America.

Western North American monarchs also migrate long distances, although their migration is shorter than monarchs in eastern North America. Whereas eastern monarchs may fly well over 2,000 mi (3,000 km) to reach the Mexican overwintering sites, western monarchs reach the California coast by flying approximately 300 mi to 1,000 mi (500 km to 1,600 km) (Yang et al. 2016, p. 1002; Edwards et al. 2023, p. 5). Western monarchs occupy warmer climates throughout the summer to include the farther reaches of their range while they continue to breed in the hotter regions (expand their range). Eastern monarchs, in contrast, follow more of a stepping-stone path into the northern States, vacating areas as they warm and recolonizing their range.

Western North American migratory monarchs occupy as much as 30 percent of the total area occupied by monarch butterflies in North America (Dilts et al. 2019, p. 11). Western monarchs use ecologically different breeding, migrating, and overwintering habitats (Brower et al. 1995, p. 542) compared to monarchs in eastern North America. Differences in breeding habitat include climate (Zalucki and Rochester 2004, pp. 220-221) and availability and abundance of native nectar and native milkweed plants (Borders and Lee-Mäder 2015, entire). The West is generally hotter and drier than the East, and the milkweed and nectar resources used by monarchs in the West and East differ (Dilts et al. 2019, entire). In the fall, western monarchs migrate from Canada and States west of the Rockies to overwintering groves located primarily along the California coast south into Baja California, Mexico (Jepsen and Black 2015, pp. 147-156). Roosting tree species used by western monarchs for overwintering are different than those used by the eastern population and include blue gum eucalyptus, Monterey pine, and Monterey cypress (Griffiths and Villablanca 2015, pp. 43-44). The western population has fewer monarchs, spread out among hundreds of overwintering sites compared to fewer than 20 sites in Mexico for the eastern population (Jepsen and Black 2015, pp. 147-156; Vidal and Rendón-Salinas 2014, entire).

Migratory monarchs in eastern and western North America may also contribute unique phenotypic variation in wing morphology/coloration, lipid reserves, and reproductive behavior (Talla et al. 2020, pp. 2572-2573; Freedman and Dingle 2018, p. 66; Davis 2009, p. 3; Brower et al. 1995, p. 542; Herman et al. 1989, pp. 52-54; 56-57). Compared to monarchs in western North America and to nonmigratory monarchs in southern Florida, eastern North American monarchs have lower rates of infection by the protozoan parasite
OE
(<10 percent; Altizer et al. 2000, p. 131), which may be due in part to their long-distance migration (Bartel et al. 2011, p. 348). Migratory monarchs in the West have
OE
infection rates (averaging 5-30 percent) that are lower than most nonmigratory populations but higher than the rates of infection in migratory monarchs in eastern North America (Altizer and de Roode 2015, p. 91).

Resident (nonmigratory) monarchs in North America now live in areas where milkweed availability and climate permit year-round breeding, and thus are able to reside continually without migrating. These nonmigratory monarchs in southern Florida are genetically distinct from the migratory North American monarchs, although the southern Florida population gets an annual influx of individuals from the eastern migratory monarch population (Knight and Brower 2009, p. 821; Zhan et al. 2014, p. 322). Also, some monarchs remain or become reproductively active and breed throughout the winter along the Gulf Coast, the southern Atlantic Coast, and the southern Pacific Coast (Howard et al. 2010, p. 3; Satterfield et al. 2016, p. 346). These monarchs are more likely to be infected with
OE
(Satterfield et al. 2016, p. 347; 2018, p. 1676), and there is some question of whether some of the offspring of these individuals might emerge in diapause and continue to Mexico or California overwintering sites later in the season (Batalden and Oberhauser 2015, p. 223). The best available information, including the quantity and quality of the habitat, indicates that the total number of resident monarchs appears to be quite small relative to the North American migratory populations that overwinter in Mexico and California. Some experts consider the resident monarch populations demographic sinks (see Crone and Schultz 2021, p. 1536), requiring continual influxes of monarchs from migratory populations to sustain them.

Nonmigratory Florida monarchs experience some of the highest recorded
OE
infection rates compared to other monarchs worldwide and particularly high rates compared to migratory eastern and western North America monarch infection rates (75-100 percent average infection rates in Florida vs. 5-30 percent infection rates in the western North American population and less than 10 percent infection rates in the eastern North American population; Altizer and de Roode 2015, p. 91). This may be due both to their inability to escape infected habitat, as well as the nonmigratory behavior not leading to any migratory culling (the removal of

less fit individuals from a population due to their inability to migrate) (Bartel et al. 2011, entire). While we assume most monarchs found in southern Florida are nonmigratory, it is possible that some are migratory (Dockx et al. 2023, pp. 314-317).

In the West, the population of migratory monarchs has declined from several million butterflies in the 1980s (Schultz et al. 2017, p. 345) to current levels (figure 1b). Concurrently in the West, a portion of nonmigratory monarch butterflies in urban gardens has been growing (Crone and Schultz 2021, entire). The increase in numbers of these nonmigratory monarchs do not seem to make up for the decline of the migratory population (Crone and Schultz 2021, entire). Additionally, the nonmigratory portion also probably lacks the demographic capacity to expand its range inland during summer months. Nonmigratory monarch butterflies build up high levels of
OE,
at least in part because of a lack of migratory culling and migratory escape (Satterfield et al. 2015, pp. 4-5). In California, about 8 percent of migratory monarch butterflies are infected with
OE,
compared to about 75 percent of nonmigratory individuals (Satterfield et al. 2016, p. 346).
OE
-infected monarch butterflies have both lower survival and lower egg-laying rates and produce only about 0.8 adult daughters per female on average (Crone and Schultz 2021, Supplement S4). While this rate of increase is enough for nonmigratory monarch butterflies to persist in urban areas, it does not allow them to expand to other western States (Crone and Schultz 2021, p. 1536). Additional information on the genetic structuring of monarchs outside of continental North America is available in the SSA report (Service 2024a, appendix 2).

In order to better understand the population dynamics and contributions from the nonmigratory populations, we are requesting additional information on resident monarchs in southern Florida, the Gulf Coast, the southern Atlantic Coast, and the southern Pacific Coast. To submit information, see the Information Requested section.

Threats

We have little to no information on positive or negative influences acting upon monarchs occurring outside of the eastern and western North American populations. There is limited information on predation, parasitism, and disease outside of eastern and western North American populations. Given this limited information, we were unable to ascertain to what extent predation, parasitism, and disease impact the monarch populations outside of the eastern and western North American populations. Similarly, while data suggest global use of insecticides is increasing, we are unable to estimate the degree of overlap with monarch populations and thus derive a credible projection of impact on the monarch populations outside of the eastern and western North American populations.

The primary drivers affecting the health of the two North American migratory populations are loss and degradation of breeding, migratory, and overwintering habitat (from past conversion of grasslands and shrublands to agriculture and widespread use of herbicides; logging/thinning at overwintering sites in Mexico; urban development, senescence, and incompatible management of overwintering sites in California; and drought); exposure to insecticides; and effects of climate change. Many other influences to monarchs were evaluated, including disease, parasitism, captive rearing, collection, impacts of tourism at overwintering sites, invasive swallow-wort plants, vehicle mortality, and natural catastrophes. However, these other potential influences were not determined to be key population drivers (Service 2024a, pp. 109-111).

Because the conversion of grasslands to agriculture was a key driver for past population declines, current and future activities that may remove milkweed and nectar resources within the breeding and migratory range, but that do not result in conversion of native or naturalized grassland, shrubland, or forested habitats and do not result in significant population-level mortality, are, therefore, not considered key population drivers. These include habitat restoration and management activities, livestock grazing and routine ranching activities, routine agricultural activities and conservation practices, fire management, silviculture and forest management, management of habitat on residential and other developed properties, and vegetation management activities when monarchs are not present. Given that so much milkweed has been lost historically and that monarchs are impacted by the ongoing effects of this past habitat loss and degradation, we need an approach that encourages landowners to add and maintain habitat in order to achieve a significant increase in the availability of milkweed and nectar resources. We expect localized removal of milkweed and nectar plants will be outweighed by an overall addition of these resources across the landscape, making broadscale public support for monarch conservation vitally important. For example, landscape-scale habitat restoration and management activities that provide for the habitat needs of monarch butterflies (
e.g.,
mowing, haying native rangeland, prescribed and cultural burning, and control of invasive plants or noxious weeds) may remove milkweed and could result in loss of monarchs in the short term but would also increase the overall quality and quantity of breeding habitat, which is likely to benefit monarch populations in the long term. We are requesting information on both our identified key threats as well as other potential threats to the species, such as collisions with wind turbines (see Information Requested). The key influences for monarch butterflies are discussed below.

Availability, Distribution, and Quality of Breeding Habitat and Migratory Habitat

The availability of milkweed is essential to monarch reproduction and survival. Reductions in milkweed are cited as a key driver in monarch declines (Brower et al. 2012, p. 97; Pleasants and Oberhauser 2013, p. 141; Inamine et al. 2016, p. 1081; Thogmartin et al. 2017a, p. 12; Waterbury and Potter 2018, pp. 42-44; Saunders et al. 2019, p. 8612).

A majority of the past milkweed loss has occurred in agricultural lands, where intensive herbicide usage for weed control has resulted in widespread milkweed eradication. More than 860 million milkweed stems were lost in the Midwest between 1999 and 2014, a decline of almost 40 percent (Pleasants 2017, p. 48). Approximately 89 and 94 percent of corn and soybean crop acreages, respectively, are planted as glyphosate (herbicide)-tolerant crops (USDA 2018, unpaginated). When glyphosate is sprayed in or near fields where crops are produced, it kills the milkweed without harming the glyphosate-tolerant crops. Glyphosate use in western agricultural lands has also increased dramatically since the 1990s, especially within the Central Valley of California, Snake River Plain of Idaho, and the Columbia River Basin, which spans the border between Washington and Oregon (USGS NAWQA 2017, unpaginated; Waterbury and Potter 2018, p. 42). As weed species develop increasing resistance to glyphosate, other herbicide (
e.g.,
dicamba)-tolerant crops are developed, which can lead to a corresponding increase in use of those herbicides. Accordingly, herbicide impacts to milkweed and nectar plants will continue to impact monarch resources in agricultural lands.

Milkweed has also been lost on the landscape through development and conversion of grasslands (Lark et al. 2015, pp. 3-4). Between 2008 and 2016, a total of 4.9 million acres of grassland were converted to new cropland, including up to 3 million acres of Conservation Reserve Program land (Lark et al. 2015, p. 5). Past loss of agricultural milkweeds in the Midwest has resulted in an estimated 81 percent decline in monarch production, in part because monarch egg densities were higher on milkweed in agricultural fields (3.89 times more eggs than on non-agricultural milkweed; Pleasants and Oberhauser 2013, pp. 139-140). This situation particularly impacts the eastern monarch population because more Mexico overwintering monarchs originate from the Midwest crop belt region than any other region (with estimates ranging from 38 percent to over 85 percent of all overwintering monarchs originating from the Midwest; Wassenaar and Hobson 1998, pp. 15438-15439; Flockhart et al. 2017, p. 2568). Development and conversion of grasslands will continue to impact monarch resources in agricultural lands.

Losses of nectar sources have also been implicated as a potential key driver in monarch declines (Inamine et al. 2016, p. 1081; Thogmartin et al. 2017a, p. 12; Saunders et al. 2019, p. 8612). Losses of nectar resources are due to the same stressors identified above for milkweed resources. Additionally, with a warming climate, drought impacts may affect the availability of nectar sources, especially in the western population and in the migratory bottleneck through Mexico and into Texas for the eastern population (see
Climate Change Effects,
below).

Our future analyses forecast a range of projections for future nectar and milkweed throughout the monarch butterfly's range. While these breeding and migratory habitat resources show signs of regional stabilization or increase, even the best case scenario increases are less than the amount of milkweed and nectar resources that have been lost. The reduced amount of breeding and migratory habitat continues to negatively impact the viability of the migratory North American monarchs. Monarch conservation needs an approach that encourages and incentivizes landowners to add milkweeds and nectar resources and implement actions to maintain this habitat. Unlike with some at-risk species, the monarch's general habitat usage and wide distribution allow a broad range of landowners to participate in actions to support the species. Conservation for the species can occur on land parcels ranging from quite small to very large, including gardens, parks, grasslands, agricultural areas, and more.

Availability, Distribution, and Quality of Overwintering Habitat

Both western and eastern monarchs rely on the microclimate provided by the trees at their overwintering sites (Leong et al. 2004, entire; Williams and Brower 2015, entire). Loss of trees occurs at overwintering sites in Mexico primarily through small- and large-scale logging, storms, and an increasingly unsuitable climate (see
Climate Change Effects
below). Most overwintering sites used by eastern monarchs occur within the Monarch Butterfly Biosphere Reserve (Reserve), a 139,019-ac (56,259-ha) protected area in Central Mexico. Within this area, logging is banned within the 33,485-ac (13,551-ha) core zone (Ramírez et al. 2015, p. 158). However, logging has continued to occur both legally (including salvage logging allowed after storms) and illegally at multiple colonies within the Reserve (Vidal et al. 2014, pp. 180-185; Brower et al. 2016, entire).

Logging was estimated in the core zone of the Reserve from 2002 through 2012 (Vidal et al. 2014, p. 180). Within this period, 5,384 acres (2,179 ha) of core zone were either deforested (less than 10 percent canopy cover remained; 3,099 ac (1,254 ha)) or degraded (a decrease in canopy cover; 2,286 ac (925 ha)). Most of these losses were attributed to illegal logging (5,083 ac (2,057 ha)), with the remaining 301 acres (122 ha) lost due to floods, drought, strong winds, and fire. Current estimates of forest loss throughout the Reserve vary from 0-2.4 percent per year (Ramírez et al. 2015, p. 163). While anti-logging and reforestation efforts are underway (López-García 2022, p. 245), logging is still ongoing within the Reserve (Brower et al. 2016, entire). Although clearcutting of forests destroys habitat directly, thinning of the forest also changes the microclimate needed by overwintering monarchs, making them more susceptible to winter mortality (Brower et al. 2011, p. 43).

Western monarch overwintering habitat along the Pacific Coast has been subject to loss through various forms of development, particularly urban development (Sakai and Calvert 1991, p. 149; Frey and Schaffner 2004, p. 172). Since the 1980s, development has caused the loss of at least 63 known California overwintering sites (Sakai and Calvert 1991, pp. 148-49, Meade 1999, p. 97-100, Xerces Society 2024a, unpaginated). Habitat alteration, such as through natural tree senescence and improper grove management, can also alter the microclimate of the western overwintering sites, leading to less suitable habitat conditions (Jepsen et al. 2015, p. 17). Many other stressors can work alone or in tandem on the western overwintering sites, including disease and pests that impact the trees used for overwintering. Fire is also a threat, both indirectly through habitat loss and directly to overwintering monarchs (Pelton et al. 2016, pp. 28, 32). Drought in the West can further exacerbate the stressors on the western overwintering sites (see
Climate Change Effects
below).

Insecticide Exposure

Insecticides are pesticides with chemical properties that are designed to kill insects and most are non-specific and broad-spectrum in nature. That is, insects exposed to these insecticides are susceptible to mortality or sublethal effects. Furthermore, the larvae of many lepidopterans (
i.e.,
insects in the order that includes butterflies and moths) are considered major pest species, and insecticides are tested specifically on this taxon to ensure that they will effectively kill individuals at application rates indicated on product labels. Even though monarchs are not the target species of these products, they may be exposed to insecticides where they are applied and in areas beyond the insecticide application sites due to drift (Olaya-Arenas and Kaplan 2019, p. 1; Halsch et al. 2020, p. 3).

Insecticide impacts to monarchs are primarily influenced by the extent to which monarchs are exposed to insecticides throughout their range. Although insecticide use is most often associated with agricultural production (
e.g.,
between 2005 and 2012, 60 percent of insecticides applied occurred on agricultural lands; EPA 2017, p. 11), any habitat where monarchs are found may be subject to insecticide use. Insecticides can be used for insect pest control anywhere there is a pest outbreak or for general pest prevention. Homeowners may treat yards and gardens to protect plants from pests or purchase plants from nurseries that sell plants pre-treated or grown from seeds treated with insecticides as ornamentals. Natural areas, such as forests and parks, may be treated to control for insects that defoliate, bore into wood, or otherwise damage trees. Outbreaks of pests, such as spongy moths, mosquitoes, Mormon crickets, or grasshoppers, may trigger insecticide treatments over large areas to control populations. Use of insecticides in vector control, especially pyrethroids and organophosphates, may be significant in areas of the country where mosquitoes pose a public health threat

or reach nuisance levels. The most widely used classes of insecticides include organophosphates, pyrethroids, and neonicotinoids. Studies looking specifically at dose-response of monarchs to neonicotinoids, organophosphates, and pyrethroids have demonstrated monarch toxicity at product label application rates and field concentration levels (
e.g.,
Krischik et al. 2015, entire; James 2019, entire; Krishnan et al. 2020, entire; Bargar et al. 2020, entire).

Insecticides are a threat to monarchs based on their mode of action to target insects and their potential exposure to monarchs. Monarchs can be exposed to liquid insecticides from direct spray at the time of application, contact with vegetation contaminated with an insecticide, or ingestion of leaves or nectar contaminated with insecticide following a spray. This exposure can occur on the site of application or in adjacent areas as a result of insecticide drift. Biopesticides, another type of insecticide, generally affect only the target pest and closely related organisms. These products contrast with broad-spectrum and conventional insecticides that target all insects. The primary identified biopesticide exposure threat to monarchs is limited to the liquid application of certain types of
Bacillus thuringiensis
(
Bt
) that are active against lepidopterans and often used to control caterpillar pests (such as spongy moth).

Monarchs may also be exposed to insecticides in forms other than liquids, such as those that have been systemically incorporated into plant tissues on which monarchs feed (
e.g.,
milkweed leaves, flowers, pollen, and nectar) or dust that has drifted off treated seeds at the time of planting. Numerous types of insecticides may be incorporated into plants systemically or used to treat seeds, including neonicotinoids. However, there are limited data to suggest that insecticides used in this manner achieve concentrations likely to result in negative effects to monarchs. Thus, while monarchs may be exposed to insecticide residues from these application methods, the available information indicates that negative effects from insecticide residues are likely minimal. For insecticides applied in solid forms such as granules, no exposure is expected to monarchs; thus, negative effects from application of pesticides in solid forms are unlikely.

Herbicides and fungicides are used within or near areas where monarchs may be present; however, only a subset of these have data showing direct negative effects to insects (
i.e.,
as opposed to insecticides, where all pesticides within this class will cause negative effects to insects). As such, the information indicates the negative effects to monarchs from exposure to herbicides and fungicides is likely minor compared to the direct effects of exposure to insecticides.

See Appendix 5—Supplementary Information About Pesticides in the monarch butterfly SSA report (Service 2024a, pp. 123-146) for further discussion of the risk of pesticides to the monarch, including data, references, and supporting information.

Climate Change Effects

Climate change can affect monarchs both directly and indirectly (Nail and Oberhauser 2015, entire) on overwintering, migratory, and breeding grounds. Increasing storm frequency in the Mexican overwintering colonies can lead to catastrophic (up to 80 percent) mortality through the freezing temperatures that accompany these storms (Anderson and Brower 1996, p. 112; Brower et al. 2004, entire). Precipitation is predicted to increase during the winter when monarchs are present in Mexico (Oberhauser and Peterson 2003, p. 14067). Severe drought can cause tree loss and degradation, decreasing the availability and quality of overwintering roosting habitat in California as well (Pelton et al. 2016, p. 29). Eucalyptus trees, the dominant tree species in many of the groves, are drought sensitive and become vulnerable to infestation by insect borers when they are stressed, which can exacerbate tree loss in these groves (Marcar et al. 1995, p. 46; Paine and Millar 2002, p. 148). Eucalyptus loss and degradation reduces availability of roosting habitat, lessens wind protection, and eliminates the primary overwintering source of nectar at many sites. Other dominant trees, such as Monterey pines and Monterey cypress, are more resistant to drought, but are the primary species in fewer than 25 percent of overwintering sites. Although overwintering grounds are widespread, drought could be equally as widespread, such that it could occur throughout many or most of the overwintering sites simultaneously. Given the above, extreme drought at overwintering sites poses a catastrophic risk for the western monarch population.

Monarchs need a very specific microclimate at their overwintering sites not just to avoid storm mortality, but also to avoid early fat depletion. Changing precipitation patterns and temperatures may influence the microclimate needed by overwintering monarchs (Williams and Brower 2015, p. 116). For example, current modeling of the monarch's fundamental niche predicts the loss of 38.6 to 69.8 percent of current suitable habitat within the Reserve due primarily to expected temperature changes (Zagorski 2016, p. 17). In western North America, climate change is predicted to cause a significant change in the distribution of overwintering monarchs in coastal California (Fisher et al. 2018, p. 10). While modeling projections suggest an inland and upslope displacement of suitable overwintering conditions by the year 2050, the best available evidence does not indicate that monarchs would move to or use these upslope areas (Fisher et al. 2018, pp. 10, 13-14).

In addition to the direct impact of climate change on overwintering monarchs, the Mexico overwintering sites are predicted to be less suitable for oyamel fir trees, the predominant monarch roosting tree. The overwintering sites are predicted to become increasingly warm throughout the year, potentially making 50 percent or more of the sites unsuitable for oyamel fir trees in 2030 (Sáenz-Romero et al. 2012, p. 102; Ramírez et al. 2015, p. 167). Widespread drought is similarly likely to negatively impact trees in the western overwintering areas both directly and indirectly due to increased susceptibility to pests (Paine and Millar 2002, p. 148).

Direct effects of climate change, particularly increasing temperatures, may impact monarch fecundity (reproductive rate) (Oberhauser 1997, pp. 168-169), mating success (Solensky and Oberhauser 2009, p. 333), and survival during migration and while overwintering (Masters et al. 1988, entire; Alonso-Mejía et al. 1997, entire). Laboratory studies indicate optimal temperatures for monarch range between 81-84 °F (27-29 °C) with sublethal effects beginning around 86-97 °F (30-36 °C) range and an upper lethal thermal limit of 108 °F (42 °C) (Zalucki 1982, p. 243; York and Oberhauser 2002, p. 294; Zalucki and Rochester 2004, p. 225; Nail et al. 2015, p. 101). Research also indicates that cooler nighttime temperatures provide respite from elevated daytime temperatures and are important in allowing monarchs to survive temperature stress (Nail et al. 2015, p. 104). Temperatures consistently above 91-95 °F (33-35 °C) are unsuitable for monarchs and may account for their general absence from the southern U.S. after spring (Malcolm et al. 1987, p. 78; Zalucki and Rochester 1999, pp. 155-157).

High temperatures and drought conditions may be particularly impactful during the crucial spring migration (Taylor 2020, pers. comm.). Spring temperatures and precipitation in the southern U.S. portion of the migratory range have been modeled to have a high relative importance affecting summer population size of eastern monarchs (a larger impact than compared to summer weather, summer herbicide use in cropland, and late-winter population size) (Zylstra et al. 2021, p. 1443). However, considerable uncertainty remains regarding the extent, intensity, and biological impacts of climate change during spring migration (Neupane et al. 2022, pp. 10-14). While increased temperatures may reduce monarch habitat in some areas, the climatically suitable niche for monarchs may increase northward, potentially increasing their summer breeding grounds if both monarchs and milkweed are able to adapt and track this niche northward (Lemoine 2015, pp. 10-17).

A warming climate may influence breeding habitat by altering suitable locations for both monarchs (Batalden et al. 2007, pp. 1369-1370) and their milkweed host plant (Lemoine 2015, entire). Nectar resources during migration may be reduced under climate conditions (decreased precipitation) projected for south-central Texas (Saunders et al. 2019, p. 8612). Drought may also influence the amount and availability of nectar needed for migrating butterflies (Brower et al. 2015, entire; Stevens and Frey 2010, p. 740; Espeset et al. 2016, p. 826). Widespread drought caused by climate change is expected to increase into the future (IPCC 2023, p. 69) negatively impacting monarchs and their habitat.

Sea level rise is increasing due to climate change. These rising sea levels are likely to impact monarch populations in coastal areas (
e.g.,
along the Gulf Coast) and low lying islands through loss of habitat (TBCSAP 2015, entire; IPCC 2023, p. 69).

Climate change may additionally impact monarchs in ways that are more difficult to measure. This may include phenological mismatch (
e.g.,
timing of milkweed and nectar sources not aligning with monarch migration; Thogmartin et al. 2017a, p. 13) or range mismatch with associated species. For example, a change in environmental suitability could cause a range shift for monarch predators and parasitoids, increasing or decreasing their overlap with the monarch's range (McCoshum et al. 2016, p. 229-233).

Conservation Efforts and Regulatory Mechanisms

While many factors have been implicated in the decline in monarch populations, the loss of milkweed and nectar resources (
i.e.,
breeding and migratory habitat) has been targeted as the threat that can be most easily addressed through conservation efforts. Because of the monarch butterfly's general habitat use and wide distribution, all sectors of society, including the general public, have an opportunity to participate in a broad range of conservation efforts throughout the species' range. Protection, restoration, enhancement, and creation of habitat is a central aspect of recent monarch conservation strategies, thus highlighting the importance of restoring and enhancing milkweed and nectar resources (Oberhauser et al. 2017, pp. 56-58; Pleasants 2017, p. 43; Thogmartin et al. 2017a, pp. 2-3; MAFWA 2018, p. 52; Pelton et al. 2019, pp. 4-5, WAFWA 2019, p. 41). Management at overwintering sites in California has also been targeted to improve the status of western North American monarch butterflies (Pelton et al. 2019, p. 4; WAFWA 2019, pp. 37-40). We are not aware of conservation actions for the populations outside of eastern and western North America, but conservation measures for the eastern and western North American populations are described below.

Major overarching landscape-level conservation plans and efforts include the Mid-America Monarch Conservation Strategy developed by the Midwest Association of Fish and Wildlife Agencies (MAFWA) and the Western Monarch Butterfly Conservation Plan developed by the Western Association of Fish and Wildlife Agencies (WAFWA). The Mid-America Monarch Conservation Strategy established a goal of adding 1.3 billion stems of milkweed on the landscape by 2038 (MAFWA 2018, p. 42). The 1.3-billion-stem goal is an estimated target for adding enough breeding and migratory habitat to support 14.8 ac (6 ha) of forest occupied by overwintering monarchs for the eastern North American population (Thogmartin et al. 2017b, pp. 2-3). Twenty-nine States—including Arkansas, Connecticut, Delaware, Illinois, Indiana, Iowa, Kansas, Kentucky, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Nebraska, New Hampshire, New Jersey, New York, North Dakota, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Dakota, Texas, Vermont, Virginia, West Virginia, and Wisconsin—have agreed to participate in the effort to reach the 1.3-billion-stem goal, which will also need contributions from multiple sectors of society, including private landowners, agricultural and nongovernmental organizations, rights-of-way organizations, and Federal, State, and local governments.

The Western Monarch Butterfly Conservation Plan currently encompasses the States of Arizona, California, Idaho, Nevada, Oregon, Utah, and Washington, which comprise the core of the western monarch range (WAFWA 2019, p. 3). The plan includes short-term goals of: (1) protecting and managing 50 percent of all currently known and active monarch overwintering sites, including 90 percent of the most important overwintering sites by 2029; and (2) providing a minimum of 50,000 additional acres of monarch-friendly habitat in California's Central Valley and adjacent foothills by 2029. It also includes overwintering and breeding habitat conservation strategies, education and outreach strategies, and research and monitoring needs. Many land managers who oversee overwintering sites in California have developed and implemented grove management strategies or have included monarch groves in their general management plans. Conservation efforts in California's Central Valley currently amount to nearly 9,000 ac (3,600 ha). As of September 2024, State agencies had implemented milkweed restoration efforts on over 8,780,404 ac (3,553,303 ha), adding more than an estimated 546 million milkweed stems to the landscape nationwide.

In early 2020, the Nationwide Candidate Conservation Agreement for Monarch Butterfly on Energy and Transportation Lands: An Integrated Candidate Conservation Agreement with Assurances (CCAA) and Candidate Conservation Agreement (CCA) was finalized and is contributing to MAFWA Strategy and WAFWA Plan goals. Under this agreement, energy and transportation entities are providing habitat for the species along energy and transportation rights-of-way corridors across the country. Participants carry out conservation measures to reduce or remove threats to the species and create and maintain habitat annually. In exchange for implementing voluntary conservation efforts and meeting specific requirements and criteria, those businesses and organizations enrolled in this CCAA/CCA receive assurance from the Service that they will not have to implement additional conservation measures should the species be listed. The goal of this CCAA/CCA is

enrollment of up to 26 million acres of land in the agreement, providing more than 300 million additional stems of milkweed (Rights-of-Way as Habitat Working Group 2020, p. 3). As of June 2024, this CCAA/CCA had 57 enrollees with over 981,812 ac (397,325 ha) of enrolled habitat.

Many conservation efforts implemented under Federal, Tribal, State, or other programs, such as the Farm Service Agency's Conservation Reserve Program; the Natural Resource Conservation Service's Environmental Quality Incentives Program, Agricultural Conservation Easement Program and Conservation Stewardship Program; and the Service's Partners for Fish and Wildlife Program, are expected to contribute to the overarching habitat and population goals of the MAFWA Strategy and WAFWA Plan. Smaller conservation efforts, such as pollinator gardens, implemented by local governments, nongovernmental organizations, private businesses, and interested individuals will also play an important role in reaching habitat and population goals established in the MAFWA Strategy and WAFWA Plan.

Many land managers who oversee overwintering sites in California have developed and implemented grove management strategies or have included monarch groves in their general management plans. Many others are in the process of developing grove management plans. As of January 2024, grove management plans are being implemented at no fewer than 24 overwintering sites and are currently being developed for at least a dozen more. Management and restoration of these sites may include activities such as replacing dead trees, modifying canopy structure, planting fall- and winter-blooming shrubs as nectar sources, and addressing monarch predation issues (Jepsen et al. 2017, entire).

The Service developed the Monarch Conservation Database (MCD) to capture information about monarch conservation plans and efforts to inform the listing decision. As of September 2024, the MCD had 145,455 complete monarch conservation effort records that have a status of completed, implemented, or planned since 2014 and 126 monarch conservation plans. These efforts constitute a total of 10,457,316 ac (4,231,926 ha) of land area in the United States (10,246,876 ac (4,146,764 ha) and 178,920 ac (72,406 ha) in the eastern and western populations, respectively) enhanced or created for monarchs, with the most common conservation effort being direct planting of milkweed and other nectar resources (note that these values include all completed, implemented, and not yet completed efforts; completed and implemented efforts to date total 7,415,731 ac (3,001,040 ha) nationally). These conservation efforts are increasing the amount of milkweed and nectar resources on the landscape. However, additional milkweed and nectar resources are required, as initial estimates of the amount of acreage needed to reach the 1.3-billion-stem goal within the MAFWA Strategy planning area were around 20 million acres.

In addition to conservation measures for monarch butterflies, there are also mechanisms that regulate direct and indirect threats to the species. One such mechanism is the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), which provides for Federal regulation of pesticide distribution, sale, and use. The term pesticide includes insecticides, which can directly kill monarchs upon exposure, and herbicides that can indirectly affect monarchs by impacting their milkweed and nectar resources. All pesticides distributed or sold in the United States must be registered (licensed) by the U.S. Environmental Protection Agency (EPA). Before the EPA registers a pesticide under FIFRA, the applicant must show, among other things, that using the pesticide according to specifications “will not generally cause unreasonable adverse effects on the environment.”

In 2021, the EPA began developing a comprehensive, long-term approach to meeting its section 7(a)(2) obligations for FIFRA actions (EPA 2022, unpaginated). As part of that work, the EPA in August 2024 finalized its first-ever strategy to identify which agricultural uses of conventional herbicides impact listed species and how to determine the amount and location of mitigation measures for those herbicides, hereon referred to as the Herbicide Strategy (EPA 2024a, entire). The strategy covers listed plants and listed animals whose survival depends on listed plants, including many insects that depend on plants for food. The EPA has begun applying the strategy to the registration of new pesticide registrations (EPA 2024d, unpaginated) and expects to apply the strategy when the agency reevaluates the registration of existing pesticides every 15 years.

Similarly, in July 2024 the EPA released its draft Insecticide Strategy, which proposes a framework to identify which agricultural uses of conventional insecticides impact listed species and how to determine the amount and location of mitigation measures for those insecticides (EPA 2024b, entire). The EPA has agreed to finalize the Insecticide Strategy by March 31, 2025, and then expects to apply it when the agency registers a new insecticide or reevaluates an existing one. The EPA is also pursuing other initiatives to minimize pesticide impacts on listed species, including through section 7(a)(2) consultations with FWS on individual pesticide active ingredients and through its Vulnerable Species Action Plan, which identifies mitigation measures for listed species particularly vulnerable to pesticides (EPA 2024c, entire).

Under the authority of the Plant Protection Act, the U.S. Department of Agriculture (USDA) regulates the importation and movement of plant pests, which covers plant-feeding insects such as the monarch butterfly. Under this regulation, a permit from USDA-Animal and Plant Health Inspection Service (APHIS) is required for the importation, interstate movement, and environmental release of butterflies, including monarchs. However, multiple States, including Alaska, Arizona, Connecticut, Hawaii, Montana, and Nevada, plus Puerto Rico, the Virgin Islands, and Guam, prohibit importation and/or interstate movement of monarchs for the purpose of environmental release. In addition, APHIS does not allow the movement of monarchs across the Continental Divide for environmental release.

Despite these conservation efforts and regulatory mechanisms, the continued threats facing the monarch butterfly have not been ameliorated.

Cumulative Effects

We note that, by using the SSA framework to guide our analysis of the scientific information documented in the SSA report, we have analyzed the cumulative effects of identified threats and conservation actions on the species. The best available scientific information indicates that there are synergistic and cumulative interactions among the factors influencing monarch butterfly viability. For example, climate change can impact the monarch directly through increased temperatures and can also impact other potential threats to the species, including habitat availability, disease, and predators. We incorporate the cumulative effects into our SSA analysis when we characterize the current and future condition of the species. To assess the current and future condition of the species, we evaluate the effects of all the relevant factors that may be influencing the species, including threats and conservation

efforts. Because the SSA framework considers not just the presence of the factors, but to what degree they collectively influence risk to the entire species, our assessment integrates the cumulative effects of the factors and replaces a standalone cumulative-effects analysis.

Current Condition

Eastern North American Population

Based on the past annual censuses, the eastern North American population has been generally declining over the last 30 years (figure 1a). Because it is not possible to count individual monarchs at the overwintering sites in Mexico, the size of the eastern North American population is measured based on the area occupied by clustered monarchs. Although the numbers at the overwintering sites have declined, we did not find a corresponding change in the spatial extent of the population during the breeding season. We developed a population model that incorporated the current size, growth rate, and year-to-year variability of the eastern North American population. The results indicate that the probability of the population abundance reaching the point at which extinction is inevitable (“probability of extinction”) in is less than 10 percent within 10 years (Service 2024a, p. 29). The probability of extinction does not account for risks from catastrophic events (discussed below in Future Condition).

Western North American Population

Based on the past annual censuses, the western North American population has been generally declining over the last 27 years, despite an increasing number of sites being surveyed (figure 1b). We developed a similar model for the western North American population as we did for the eastern population. Under current conditions, the risk of extinction is predicted to increase sharply over time, between 60 to 68 percent within 10 years (Service 2024a, p. 30). The probability of extinction does not account for risks from catastrophic events (discussed below in Future Condition).

EP12DE24.000

EP12DE24.001

Figure 1—(a.) Area occupied (in hectares) by eastern North American monarch butterflies at overwintering sites in Mexico. Year displayed is the beginning year for the winter (
e.g.,
2017 represents the number for the winter of 2017-2018). (Data from Rendón-Salinas et al. 2024 (p. 3).)

(b.) Survey counts showing the number of western North American monarch butterflies observed at overwintering sites (bars). The black line shows the number of sites monitored (survey effort) for a given year. (Data from the Xerces Society for Invertebrate Conservation (2024a, entire).)

Dispersed Nonmigratory Populations

Limited information is available on the status and health of monarchs outside of the North American migratory populations or regarding the positive or negative influences acting upon these populations. Based on observations of the species throughout its range, 69 of the 90 countries, islands, and island groups are currently extant. Monarch presence within the remaining 21 countries, islands, and island groups has not been confirmed since 2000, but with no evidence of extirpation, they are all presumed extant. Furthermore, we grouped occurrences into 31 populations. Of the 29 populations outside of North America, 25 are confirmed extant and the remaining 4 are presumed extant (Service 2024a, pp. 40-42). We were not able to further assess the level of resiliency of these populations.

Current Resiliency, Redundancy, and Representation

The species' redundancy is evident through its confirmed or presumed presence in all 90 of the countries, islands, and island groups where it occurred historically or to where it has dispersed. The species' adaptive capacity (representation) is evident through its presence over a large geographical range made up of 31 known populations (2 North American migratory populations and 29 nonmigratory or dispersed populations) where the climatic conditions and habitat vary widely. The species' resiliency varies between populations, with the estimated probability of extinction for the eastern migratory North American population at less than 10 percent in 10 years and 60 to 68 percent for the western migratory North American population in 10 years. For monarchs outside of the North American migratory populations, 69 of the 90 countries, islands, and island groups are considered currently extant, although we were not able to further assess the level of resiliency of these populations.

Future Condition

Future Scenarios and Catastrophic Events

North American Migratory Populations

To assess the future condition of monarch populations, we organized the key factors driving monarch population dynamics into six categories: (1) milkweed availability, (2) breeding nectar availability, (3) migration nectar availability, (4) overwintering habitat availability, (5) climate change effects, and (6) insecticide exposure. We then forecasted plausible increases and/or decreases, as appropriate, for each of these influences, using the best available information, including scientific literature and expert input. We also included conservation efforts outlined in large-scale monarch conservation plans, such as the MAFWA Strategy and WAFWA Plan. We did not evaluate these plans under the Policy for Evaluating Conservation Efforts (68 FR 15100; March 28, 2003) because these formalized conservation efforts have been implemented. Next, we combined the upper plausible limit and the lower plausible limit for each influence (changes in milkweed, nectar, and overwintering habitat; climate change; and insecticides) to form composite plausible best case and plausible worst case scenarios, respectively. Lastly, we incorporated these scenarios into the population models described in the
Current Condition
section, and forecasted population numbers to 60 years to determine the probability of extinction for both the eastern and western migratory North American populations to 2080 (Service 2024a, tables 6.1 and 6.2, pp. 45-47).

We also evaluated several potential events to determine if they were of sufficient magnitude and severity to cause a population collapse (
i.e.,
a catastrophic event). We determined that extreme storm events and widespread

drought have sufficient potential to pose a catastrophic risk to the eastern population, and widespread drought and co-occurrence of poor environmental conditions and low population abundance have sufficient potential to pose a catastrophic risk to the western population.

Dispersed Nonmigratory Populations

Due to a lack of information on current influences, we were unable to forecast future scenarios for the populations outside of eastern and western North America. However, we identified two potential catastrophic events, both of which are effects of climate change: sea level rise and lethal high temperatures.

Future Conditions

Eastern North American Population

Under both best and worst case scenarios described above, the population continues to decline (λ < 1). The greatest impact on the population occurs during the first 20 years for both scenarios; we had a slight increase in the growth rate from the current value under the best case scenario and a decrease of 4.5 percent under the worst case scenario. As expected under a declining growth rate, the probability of extinction increases over time (Service 2024a, p. 64). In 30 years, probability of extinction ranges from 24 to 46 percent. In 60 years, the probability of extinction for the eastern North American population ranges from 56 to 74 percent.

We were unable to incorporate direct effects from increasing temperatures and catastrophic risks into the population models, so we qualitatively discuss the implications of these factors on the future condition of the population. We evaluated the changes in the spatial extent and number of days with projected temperatures above lethal and sublethal thermal thresholds during critical time periods in monarch migration (Service 2024a, pp. 120-122). We assessed these changes under two future scenarios, using Representative Concentration Pathways (RCPs). RCPs reflect different levels of greenhouse gas emissions and the resulting climate change scenarios (IPCC 2014, p. 57). We used RCP4.5 and RCP8.5 scenarios projected to 2069 (Service 2024a, p. 122). Under the RCP4.5 scenario, both the spatial extent and the average number of days above 38 °C (100 °F) (the threshold for sublethal effects and moderate reductions in survival) are projected to markedly increase throughout much of the range, including the southern and northeastern portions of the eastern North American monarch breeding range. Although in the northcentral area of the breeding range, there is a projected decrease in spatial extent and the average number of days above 38 °C (100 °F). Under the RCP8.5 scenario, both the spatial extent and the average number of days above 38 °C (100 °F) have large increases throughout the entirety of the breeding range. The spatial extent and average number of days above 42 °C (107.6 °F) (the lethal threshold) are projected to increase dramatically in the southern U.S. during the same period under both scenarios. Given these results, monarch reproductive success and survival rates of the first generation of monarchs coming from the wintering grounds are likely to decline, although the extent to which these rates will decline is unknown.

Similarly, given the projected population decline described above, the eastern population will be increasingly vulnerable to catastrophic losses due to extreme storm events at the overwintering grounds and widespread droughts during the breeding season and along the migratory route. Although we cannot quantify this increased risk, the longer the eastern population remains at low population abundance, the more likely it is that catastrophic losses will occur and the greater the extinction risk for the eastern population.

Western North American Population

Under both scenarios, the population continues to decline (λ < 1). Under the best case scenario, monarchs have a slight increase in the growth rate from the current value; however, even with an increase, this was still a declining growth rate. Under the worst case scenario, the growth rate decreased to a lower rate than the current rate. As would be expected with a declining growth rate, the probability of extinction increases over time (Service 2024a, p. 66). In 30 years, probability of extinction ranges from 92 percent to 95 percent. By year 60, the probability of extinction reaches 99 percent for the western North American population.

Under the RCP4.5 scenario, increases are projected for the average numbers of days above 38 °C (100.4 °F) (38 percent) and above 42 °C (107.6 °F) (11 percent). Given this, monarch reproductive success and survival rates are likely to decline, although the extent to which these rates will decline is unknown.

Similarly, given the projections of monarch health described above, the western population is vulnerable to catastrophic losses due to both widespread drought events and the co-occurrence of poor environmental conditions and low population abundance. The risk of extinction due to these events increases the longer the population remains at the current low abundances.

Dispersed Nonmigratory Populations

We qualitatively assessed the impact due to predicted climate change effects. Fifteen of the 29 populations are classified as being “at risk” due to threats associated with climate change (6 due to sea level rise and 9 due to unsuitably high temperatures). The populations susceptible to sea level rise (Johnston Atoll, Kiribati, Marshall Islands, Nauru, Tokelau, and Tuvalu) are at risk of losing at least some of their monarch habitat; thus, redundancy could decrease with the loss of those areas. However, the best available information does not indicate if populations at risk to high temperatures will lose all or just a portion of their monarch habitat; thus, they may continue to contribute to redundancy. Each of the populations at risk due to sea level rise contains a single country, island, or island group. Therefore, in the dispersed nonmigratory populations the species will continue to have redundancy through continued presence in an estimated 84 of the 90 countries, islands, and island groups where it occurred historically or to where it has dispersed. We anticipate the species will continue to have adaptive capacity (representation) through its presence over a large geographical range where the climatic conditions and habitat vary widely.

Future Resiliency, Redundancy, and Representation

Both the eastern and western migratory North American populations become more vulnerable to catastrophic events (
e.g.,
extreme storms at the overwintering habitat) into the future resulting in lower redundancy for the species. Under plausible climate change scenarios, monarch butterflies will be exposed to unsuitably high temperatures for more days each year and over larger areas of their range in North America. Outside of the two North American migratory populations, 15 of the 29 nonmigratory or dispersed populations are at risk in the future due to threats associated with climate change (6 due to sea level rise and 9 due to unsuitably high temperatures). The populations susceptible to sea level rise are at risk of losing at least some of their monarch habitat; thus, redundancy could decrease with the loss of those areas. Rangewide, the species is likely to maintain considerable redundancy and adaptive capacity (representation)

through continued presence in an estimated 84 of the 90 countries, islands, and island groups where it occurred historically or to where it has dispersed. Despite uncertainties about resiliency at the nonmigratory and dispersed populations, the widespread distribution of monarch populations indicates that the species has low risk of becoming extirpated from multiple locations should a large-scale catastrophic event occur; thus, it is unlikely that a single catastrophic event would affect the entire species across its large range.

In the future, the resiliency of the eastern and western North American migratory populations will continue to decline. The estimated probability of extinction for the eastern migratory North American population is 56-74 percent in 60 years and greater than 99 percent for the western migratory North American population in 60 years.

Determination of Monarch Butterfly Status

Section 4 of the Act (16 U.S.C. 1533) and its implementing regulations (50 CFR part 424) set forth the procedures for determining whether a species meets the definition of an endangered species or a threatened species. The Act defines an “endangered species” as a species in danger of extinction throughout all or a significant portion of its range, and a “threatened species” as a species likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range. The Act requires that we determine whether a species meets the definition of an endangered species or a threatened species because of any of the following factors: (A) The present or threatened destruction, modification, or curtailment of its habitat or range; (B) overutilization for commercial, recreational, scientific, or educational purposes; (C) disease or predation; (D) the inadequacy of existing regulatory mechanisms; or (E) other natural or manmade factors affecting its continued existence.

Status Throughout All of Its Range

After evaluating threats to the species and assessing the cumulative effect of the threats under the Act's section 4(a)(1) factors, we determined that the primary threats affecting the monarch and its habitat are the ongoing impacts from loss and degradation of breeding, migratory, and overwintering habitat (from past conversion of grasslands and shrublands to agriculture and widespread use of herbicides; logging/thinning at overwintering sites in Mexico; urban development, senescence, and incompatible management of overwintering sites in California; and drought) (Factor A); exposure to insecticides (Factor E); and effects of climate change (Factor E).

While the monarch butterfly is historically native to North America with migratory and nonmigratory populations, monarchs have dispersed via human assistance from North America in the past two centuries and their range now includes populations throughout 90 countries, islands, and island groups where milkweed was already present or introduced. Populations exist in Central and South America, Australia, New Zealand, islands of the Pacific and Caribbean, and elsewhere (see Service 2024a, pp. 41-42). The primary threats have been affecting the resiliency of the eastern and western North American migratory populations over the last 20 years, and both populations now have lower abundances and declining population growth rates. However, in its current condition, the probability of extinction of the eastern migratory population is less than 10 percent over the next 10 years. The probability of extinction of the western migratory population over that same time period is higher (60-68 percent).

For monarchs outside of the two North American migratory populations, 69 of the 90 countries, islands, and island groups are considered currently extant. Monarch presence within the remaining 21 countries, islands, and island groups has not been confirmed since 2000, but the best available information does not indicate they are extirpated, and thus they are all presumed extant. Of the 29 populations outside of North America, 25 are confirmed extant, and the remaining 4 are presumed extant (see Service 2024a, pp. 40-42). Although we were not able to further assess the level of resiliency of these 29 nonmigratory or dispersed populations, the species' redundancy is evident through its confirmed or

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Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/fr%3A2024-28855. Public record. Not legal advice.