Monarch Butterflies in Virginia

Monarch butterfly
Monarch butterfly
Source: National Park Service

As the human population grows in Virginia, habitat is transformed and wildlife populations are affected. Clearing the forests for agriculture in the 1600-1800's, and the expansion of the suburbs in the 1900's, had the greatest impact. Some species, such as the passenger pigeon, will never be seen again. Others, such as the buffalo, will never reappear in the wild in Virginia. Some, such as the pileated woodpecker, are struggling to survive in habitat that has been fragmented by roads and fields.

Butterflies and moths, however, are likely to survive the transformation of Virginia's natural setting. These insects are able to fly to isolated pockets of habitat, including gardens planted in suburbia intentionally to attract butterflies. The species composition of the butterflies and moths of Virginia has changed as forests were replaced with fields, but future generations of Virginians may be able to enjoy some wildlife right in front yards.

A special effort is underway to save monarch butterflies (Danaus plexippus) in Virginia and across North America. The population that migrated from wintering grounds in Mexico to the Northeast declined by 80% after the mid-1990's, dropping to only 200 million in 2018. The population that wintered in Monterey and Pacific Grove, California and migrated into the Pacific Northwest dropped by 98%, from 1.2 million to just 30,000.

Monarch butterflies transition from eggs to caterpillar, then metamorphose in a chrysalis into a butterfly
Monarch butterflies transition from eggs to caterpillar, then metamorphose in a chrysalis into a butterfly
Source: National Park Service, Monarch Life Cycle

Between 1993-2001, an average of 8.7 hectares (21.5 acres) of trees were occupied by overwintering monarchs in Mexico. Between 2002-2011, that dropped to 5.7 hectares (14 acres). Between 2011-2021, the size of the forest occupied in the winter dropped even further to 2.62 hectares (5.5 acres).


Source: Nature (on PBS), Inside a Monarch Swarm

In December 2020, the US Fish and Wildlife Service determined that monarch butterflies qualified as a threatened species under the Endangered Species Act of 1973. There was a 10% chance of the main population which migrated to Mexico going extinct in the next 10 years, and a 60-68% chance of the western population migrating to California going extinct within that time.1

However, the Federal agency announced that listing was "warranted but precluded" by lack of resources. The US Fish and Wildlife Service planned to focus on higher-priority species. The International Union for Conservation of Nature officially designated the species as "endangered" in 2022, due to the 20-90% population decline over several decades.

Resource constraints were less relevant to the International Union for Conservation of Nature, while the US Fish and Wildlife Service would be responsible for work such as creating a restoration plan and designating critical habitat.2

Recovery of the monarch is facilitated by people planting native species of milkweed, particulary common milkweek (Asclepias syriaca). The caterpillars in the Spring and Summer depend upon milkweed as their food source. Other species, such as the Milkweed Tussock Moth or Milkweed Tiger Moth (Euchaetes egle), also feed on milkweeds and absorb cardiac glycosides from the plant's milky sap.

both monarch and Milkweed Tussock Moth caterpillars absorb the toxic sap from milkweed plants
both monarch and Milkweed Tussock Moth caterpillars absorb the toxic sap from milkweed plants

plants in the milkweed family, such as Asclepias syriaca, provide toxins to monarch butterflies that protect the adults from predation
plants in the milkweed family, such as Asclepias syriaca, provide toxins to monarch butterflies that protect the adults from predation

In the Fall, adult monarchs migrate through Virginia and from Virginia to the Gulf Coast before flying to Mexico. Some may choose to spend the winter in South Florida roosts.3

There is no guarantee that the migration will continue. On the West Coast, millions of monarchs living west of the Rocky Mountains used to migrate to overwinter in California at about 400 different sites. Voters approved a bond issue in 1988 to acquire the roosting sites. The California Department of Parks and Recreation, other government landowners, private corporations, and individuals sought to protect the overwintering trees from harvest, in contrast to illegal logging in Mexico's overwintering forests.

Excessive tree trimming, new development, and wildfires have damaged or destroyed sites and reduced their capacity to harbor adult butterflies. Drought has killed patches of non-native blue gum eucalyptus which had neared the end of their lifespan, but have protected monarchs for the last century. Pesticides in milkweed and plants used for nectar feeding have affected population numbers, and warming temperatures may have enticed some monarchs to become year-round residents rather than migrate south to warmer winter roosting sites.

In 2018 and 2019 the overwintering populations in California had dropped to 30,000 monarchs, with the largest number concentrating at Pismo State Beach. Population models suggested the species was at a tipping point, and in 2020 the Xerxes Society estimated the total number of migrating monarchs in California at just 2,000. That was a decline of 99.9% from the 10 million monarch butterlies that once lived in the state. The International Union for Conservation of Nature declared the migratory monarchs of North America to be an "endangered" species and added it to the Red List in 2022.

Larger populations of non-migrating monarchs may exist in San Francisco and other areas where milkweed is surviving year-round. Those populations keep the species from disappearing on the West Coast. As one professor commented regarding the San Franciso monarchs:4

Maybe they don't need to migrate anymore if it's always warm enough to live here year-round.

in the recent past, West Coast monarchs migrated and overwintered at multiple sites
in the recent past, West Coast monarchs migrated and overwintered at multiple s
Source: Xerces Society, Western Monarch Count

The decline in the overwintering population may be offset by the natural increase in monarch populations during the summer. One female can lay as many as 500 eggs. A 2022 study reported that the summer population had been steady for the last 25 years, suggesting that efforts to expand the number of milkweed plants had not enhanced the natural recovery. According to one of the study's co-authors:5

There's this perception out there that monarch populations are in dire trouble, but we found that's not at all the case... Everybody thinks monarch habitat is being lost left and right, and for some insect species this might be true but not for monarchs... If you think about it, monarch habitat is people habitat. Monarchs are really good at utilizing the landscapes we've created for ourselves. Backyard gardens, pastures, roadsides, ditches, old fields - all of that is monarch habitat.

monarch caterpillars spend time in a chrysalis to metamorphose into butterflies
monarch caterpillars spend time in a chrysalis to metamorphose into butterflies

The study's co-author was aware that populations had declined in the Midwestern breeding range, perhaps because of the herbicide glyphosate being used more widely in corn and soybean fields. In the northern part of that range, however, climate change had improved the habitat and summer breeding populations were increasing. The threat to the survival of the species was during from the southern migration and at the wintering sites, not an absence of milkweed during the summer.

His conclusion was:6

...it's not really a production problem... We don't have fewer monarchs. We have fewer monarchs reaching the wintering colonies.

solar energy moving from leaf towards butterfly
solar energy moving from leaf towards butterfly
waxy coating on milkweed leaf is no deterrent
waxy coating on milkweed leaf is no deterrent
multiple caterpillars seeking a meal
multiple caterpillars seeking a meal
caterpillar on denuded milkweed plant
caterpillar on denuded milkweed plant
monarch chrysalis with ant visitor
monarch chrysalis with ant visitor
adult soon after emerging from chrysalis
adult soon after emerging from chrysalis
adult, almost ready to fly to Mexico
adult, almost ready to fly to Mexico
empty chrysalis after adult emerged
empty chrysalis after adult emerged
(click on images for larger version)

Planting extra milkweed added another factor that disrupted the migration pattern. People interested in expanding the summer monarch population bought a non-native tropical milkweed, Asclepias curassavica, from garden stores. That milkweek reproduced and lived throughout the winters in southern Texas and along the Gulf Coast. Some monarchs adapted and established a year-round population, without migrating to Mexico for the winter.

As with the native milkweed species, the tropical milkweed is home to the protozoan parasite called OE (Ophryocystis elektroscirrha). Catrpillars ingest it along with the latex-rich juices of the milkweed plant, including the cardenolides that deter predation. Adult monarchs who start to emerge from their chrysalises carry spores of the parasite. The parasite prevents successful emergence of some adults, while those who do fly away and spread the parasite futher are weakened and die prematurely.

tropical milkweed, Asclepias curassavica, survives over the winter and allows quick reinfection of migrating monarchs with a parasite
tropical milkweed, Asclepias curassavica, survives over the winter and allows quick reinfection of migrating monarchs with a parasite
Source: Wikipedia, Asclepias

The migration cycle weeds out the sick monarchs, blocking transmission of the spores to the wintering grounds. The milkweeds are annual plants, so during the winter all of them - including the infected milkweeds - die. That interrupts the infection process. When new migrants head north, healthy monarchs encounter newly-grown milkweed plants which have not been infected yet by the parasite, and a healthy monarch population expands again.

The non-native milkweed that lives during the winter provides a year-round source of the parasite. The first generation of migrants flying north in the spring can become infected, threatening the health of the whole summer "crop" of monarchs. The Xeexes Society says:7

While milkweed is needed in large numbers to support and expand the monarch butterfly population, we do not recommend planting tropical milkweed, and further suggest milkweed of any species not be planted within 5–10 miles of monarch overwintering sites in California.

monarch caterpillars graze from the underside of leaves, providing some protection from hungry birds unfamiliar with their bad taste monarch caterpillars graze from the underside of leaves, providing some protection from hungry birds unfamiliar with their bad taste
monarch caterpillars graze from the underside of leaves, providing some protection from hungry birds unfamiliar with their bad taste

Links

monarch caterpillar feeding on milkweed
monarch caterpillar feeding on milkweed
monarch caterpillar feeding on milkweed

References

1. "Scientists scramble to learn why monarch butterflies are dying so quickly," UPI, July 22, 2019, https://www.upi.com/Top_News/US/2019/07/22/Scientists-scramble-to-learn-why-monarch-butterflies-are-dying-so-quickly/6961563481223/; "Endangered and Threatened Wildlife and Plants; 12-Month Finding for the Monarch Butterfly," Federal Register notice, US Fish and Wildlife Service, December 17, 2020, https://www.federalregister.gov/documents/2020/12/17/2020-27523/endangered-and-threatened-wildlife-and-plants-12-month-finding-for-the-monarch-butterfly; "Monarch Winter 2021–2022 Population Numbers Released," Monarch Conservation, https://monarchconservation.org/wp-content/uploads/2022/05/Monarchs-Winter-Population-21-22.pdf (last checked October 3, 2022)
2. "Endangered Species Act Listing for Monarch Butterfly Warranted but Precluded," US Fish and Wildlife Srvice, December 15, 2020, https://www.fws.gov/press-release/2020-12/endangered-species-act-listing-monarch-butterfly-warranted-precluded; "Monarch butterfly declared endangered amid declining numbers," Public Broadcasting System (PBS), July 30, 2022, https://www.pbs.org/newshour/amp/show/monarch-butterfly-declared-endangered-amid-declining-numbers (last checked October 3, 2022)
3. "Like your grandparents, monarch butterflies are now wintering in Florida," Popular Science, January 2, 2019, https://www.popsci.com/monarch-butterflies-winter-destinations-florida/; Hannah B. Vander Zanden, Carol L. Chaffee, Antonio González-Rodríguez, D.T. Tyler Flockhart, D. Ryan Norris, Marta L. Wayne, "Alternate migration strategies of eastern monarch butterflies revealed by stable isotopes," Animal Migration, Volume 5, Issue 1 (January, 2019) https://doi.org/10.1515/ami-2018-0006; "The Other Milkweed Caterpillar: Milkweed Tussock Moth, Euchaetes egle," Bug of the Week, August 25, 2014, https://bugoftheweek.com/blog/2014/8/22/the-other-milkweed-caterpillar-milkweed-tussock-moth-ieuchaetes-eglei (last checked August 31, 2022)
4. "Monarch Population in California Spirals to Another Record Low," Xerxes Society, November 30, 2020, https://xerces.org/blog/monarch-population-in-california-spirals-to-another-record-low; "The Vanishing Butterfly Groves of California," Xerces Society, January 27, 2021, https://xerces.org/blog/vanishing-butterfly-groves-of-california; "Scientists Try to Save Migratory Western Monarch Butterflies as a Mystery Unfolds," KPIX, July 23, 2021, https://sanfrancisco.cbslocal.com/2021/07/23/scientists-try-to-save-migratory-western-monarch-butterflies-as-a-mystery-unfolds/; "Why monarch butterflies, now endangered, are on the 'edge of collapse'," Washington Post, July 21, 2022, https://www.washingtonpost.com/climate-environment/2022/07/21/monarch-butterfly-endangered-iucn/ (last checked July 25, 2022)
5. "Monarch butterfly populations are thriving in North America," UGA Today, June 11, 2022, https://news.uga.edu/monarch-butterfly-populations-are-thriving/; Michael S. Crossley, Timothy D. Meehan, Matthew D. Moran, Jeffrey Glassberg, William E. Snyder, Andrew K. Davis, "Opposing global change drivers counterbalance trends in breeding North American monarch butterflies," Global Change Biology, June 10, 2022, https://doi.org/10.1111/gcb.16282 (last checked June 11, 2022)
6. "Some Monarch Butterfly Populations Are Rising. Is It Enough to Save Them?," New York Times, June 10, 2022, https://www.nytimes.com/2022/06/10/science/monarch-butterflies-mexico.html (last checked June 14, 2022)
7. "Tropical Milkweed—A No-Grow," Xerxes Society, April 19, 2018, https://xerces.org/blog/tropical-milkweed-a-no-grow; "Plan to save monarch butterflies backfires," Science, January 13, 2015, https://www.science.org/content/article/plan-save-monarch-butterflies-backfires (last checked July 17, 2022)

monarchs feed on milkweed as caterpillars, and on the nectar/pollen as adults
monarchs feed on milkweed as caterpillars, and on the nectar/pollen as adults
Source: National Park Service, Monarch Butterfly on Milkweed Plant


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