What’s causing the decline in monarch butterfly populations?

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Written by Mariecor Agravante
on Mar 27, 2020image

Habitat loss

For monarchs, habitat entails food, water and shelter, says the National Wildlife Federation (NWF) and the World Wildlife Fund (WWF). Specific to monarchs is their habitat corridor, a trek of thousands of miles from Central America’s warm regions, where they overwinter, to areas across the United States and southern Canada, where they stay for spring and summer.

In recent decades, population surveys reveal monarchs declining because of deforestation in Mexico, loss of grasslands in the Great Plains’ Corn Belt — which the Center for Biological Diversity calls “the heart of the monarch’s range” — and loss of native milkweed plants in the U.S. Such habitat losses negatively impact monarch populations as they breed, migrate and overwinter.

Habitat loss stems mainly from the deforestation of overwintering areas, climate change‘s fluctuating weather patterns, developmental sprawl, plus the conversion of U.S. grasslands into ranches and farmlands. This conversion to farmland for corn and soy has spurred the Center for Biological Diversity’s admonishment against the overuse of herbicides. These harmful chemicals poison a key player in monarch habitats, their host plant, the milkweed.

Problems with milkweed

Milkweed is vital to monarchs. They are host plants, upon which females lay eggs. Once hatched, caterpillars enjoy milkweed as a food source while they grow and develop into adulthood, a process that happens in the first month of a monarch’s lifespan. And, as adults, the butterflies feed on milkweed nectar. Several generations of offspring spawn on milkweed during spring and summer months before migration to overwintering sites even begins.

According to the NWF, “Monarch caterpillars feed exclusively on the leaves of milkweed, the only host plant for this iconic butterfly species. As such, milkweed is critical for the survival of monarchs. Without it, they cannot complete their life cycle and their populations decline.”

Interestingly, milkweed has the toxin cardenolide, which accumulates in caterpillars feeding on milkweed. When these caterpillars become adults, the cardenolides remain, protecting them from predation. Birds and predators veer away, signaled off by the toxin’s presence in the monarchs’ bright wings.

Unfortunately, milkweed loss is increasing in the destabilized landscape. Milkweed has lost considerable ground to urbanization, shifting land management practices, climate change and even herbicide misuse, like that of Roundup.

Alarming still are reports by Science magazine and Entomology Today that well-meaning gardeners have been planting the wrong species of milkweed. There are over 100 milkweed species, and not all are good for monarchs. Sadly, the tropical milkweed species Asclepias curassavica is heavily marketed because it is easier to obtain. But this invasive species is not well-suited for monarchs, yet remains the species good-intentioned gardeners are planting rather than the native milkweed species the monarchs are better adapted to. This invasive milkweed is now recognized by the Ecological Society of America as an ecological trap for monarch butterflies.

What dangers do these “wrong” species of milkweed pose for monarchs? For one, they harbor parasites, such as the protozoan parasite Ophryocystis elektroscirrha (OE), that are harmful to the monarch butterfly. These parasites debilitate monarchs, weakening them via “wing deformities, smaller body size, reduced flight performance, and shorter adult lifespans,” Entomology Today explained. Should these issues with milkweed persist unmitigated, their repercussions would continue to exacerbate the monarch butterfly population crisis.

a monarch butterfly resting on an orange flowered plant

Pesticide, insecticide and fungicide misuse

While media attention has spotlighted herbicides as a culprit, equally important is the fact that monarch butterflies are also vulnerable to pesticidesneonicotinoid insecticides and fungicides. For instance, a Purdue University Department of Entomology study, published last summer 2019 in Frontiers in Ecology and Evolution, revealed that non-target pesticides, insecticides and fungicides have wreaked havoc on monarch butterflies, even at their larval stage.

As the study elucidated, “agricultural intensification and a corresponding rise in pesticide use has been an environmental concern” that adversely affects beneficial pollinators, like the monarch butterfly. Exposure to these pesticides, insecticides and fungicides, can be from “direct contact with contaminated surfaces or spray droplets, residues remaining on the soil, and consumption via food resources such as leaves, nectar or pollen.” Just as vexing are pesticides, insecticides and fungicides “applied by aircraft.” The study emphasized the “evidence of lower abundance and/or diversity of butterflies.”

Climate change

The WWF affirms that “monarchs are highly sensitive to weather and climate. They depend on environmental cues (temperature in particular) to trigger reproduction, migration, and hibernation.” Their decline is also attributed to “the effects of an increasing frequency of extreme weather events such as drought and severe storms, and extremes in hot and cold temperatures.” No wonder then that the Environmental Defense Fund‘s Director of Conservation Studies, David Wolfe, has lamented that “The iconic and beloved North American monarch butterfly is one of the species that has difficulty adjusting to our new climate-stressed world. Its population has declined 95 percent in the last 20 years

Yet another way climate change adversely affects monarch butterflies is by disrupting their migration. These butterflies can travel between 50 and 100 miles a day, but when extreme weather sets in during migration, the entire cluster or roost is vulnerable.

“Every year, a new generation of these butterflies follows the same path forged by generations before them. The only thing guiding them on this migration is temperature telling them when they need to travel – like a biological trigger setting them in flight,” Wolfe explained. “But in recent years, the monarch’s fall south migration from Canada has been delayed by as much as six weeks due to warmer-than-normal temperatures that failed to trigger the butterflies’ instincts to move south. By the time the temperature cooled enough to trigger the migration, it’s been too cold in the Midwest and many monarchs died on their trip south.”

Even more worrisome, the Xerces Society, a nonprofit environmental group focused on invertebrates, has reported that warmer temperatures from climate change increase the toxicity of tropical milkweed by increasing cardenolide concentrations. Monarch caterpillars are only tolerant up to a threshold.

EcoWatch explained, “warmer temperatures increase the cardenolides in A. curassavica [the tropical milkweed species] to the point where they poison monarch larvae, delaying larval growth and stunting adult forewings. Native milkweed is not similarly impacted.” Hence, as invasive milkweed persists, they further harm monarch populations as temperatures rise in our current climate crisis.

a monarch butterfly on a green-leafed plant

Diseases, parasites and fungal pathogens

Emory University emphasizes that climate change affects pathogen development, parasite survival rates, disease transmission processes. What would monarch populations be susceptible to?

Bacterial and viral infections — like bacillus thuringiensis (BT), pseudomonas, the nuclear polyhedrosis virus (NPV) — are not unheard of, often turning an infected caterpillar or chrysalis into a darkened or black hue. Parasite attacks can come from tachinid flies or wasps (chalcid, trichogramma). Plus, fungal pathogens in the genus Cordyceps also attack. Each of these factors cause harm to monarch butterfly populations.

What’s Happening to the Monarch Butterfly Population?

“Something’s going on in early spring,” a professor said, and researchers are trying to solve the mystery.

Thousands of monarch butterflies gather in the eucalyptus trees at the Pismo State Beach Monarch Butterfly Grove.
Credit…George Rose/Getty Images


Western monarch butterflies spend their winters in Pismo Beach and other sites on the central California coast. A few months later, they breed in the Central Valley and as far north and east as Idaho.

But where they go in between remains an open question.

Now, a group of researchers wants the public’s help to solve that mystery.

They would like anyone who spots a monarch north of Santa Barbara this spring to snap a quick picture. The researchers — from Washington State University, Tufts University, the nonprofit Xerces Society for Invertebrate Conservation, and the University of California, Santa Cruz — need photographic evidence, a date and a location to confirm where the monarchs might be living. (Photos and information can be emailed to monarchmystery@wsu.edu or uploaded on the iNaturalist app.)

Monarch Butterfly Populations Are Plummeting



Both Eastern and Western monarch butterflies are seeing their populations plummet precipitously, worrying scientists that the future of the species is in peril, according to multiple surveys of butterfly populations.
The New York Times recently reported on efforts to track the Western monarch butterfly, which spends its winter on California’s central coast before heading off to breeding sites that covers a wide swathe from the state’s central valley all the way to Idaho. However, recently it’s been harder to find migrating butterflies.

“Something’s going on in early spring,” said Cheryl Schultz, a professor at Washington State University in Vancouver, to The New York Times. Researchers know that winter survival isn’t the issue in the short-term, she said.

The researchers do not know if butterflies are not making it to breeding sites, unable to find mates, or starving along the way. What they do know is that the population of Western monarchs was in the millions in the 1980s, down to 200,000 three years ago, and then down to 30,000 in 2018, according to The New York Times.
Matt Forister, an insect ecologist at the University of Nevada, Reno, told the The New York Times that research identified various factors in butterfly loss, including development, climate change, farming practices and widespread pesticide use by farmers and on lawns.

On the other side of the country, expect to see far fewer Eastern monarch butterflies migrating over the summer. According to a new population survey, the Eastern monarch has passed the extinction threshold, according to a new population survey by the Center for Biological Diversity.

An annual count showed that the population in 2020 dropped 53 percent from its already low 2019 numbers. “Scientists were expecting the count to be down slightly, but this level of decrease is heartbreaking,” said Tierra Curry, a senior scientist at the Center for Biological Diversity in a statement. “Monarchs unite us, and more protections are clearly needed for these migratory wonders and their habitat.”

As Mongabay reported, the National Commission of Protected Natural Areas of Mexico teamed up with WWF-Mexico, local communities and other partners to carry out an annual survey of the forest habitat covered by monarch butterflies that migrate from the U.S. and Canada. The area of forest inhabited by monarchs in Mexican forests is then used to estimate the monarch butterfly population.
The amount of land they covered was about seven acres, down from 15 acres in 2019.

The Western monarch, which is slightly smaller and darker than the Eastern monarch, has a more constrained migratory pattern. While the Eastern monarch travels from Mexico all the way to New England and Canada, the Western monarch tends to stay around California, traveling as far north as British Columbia and east as far as the Rockies, according to The New York Times. However, in recent years, it has narrowed its path and does not make it as far as Washington.

Researchers and environmental advocates point out that stemming the climate crisis, reducing pesticide use and planting pollinator gardens could help the butterflies recover.

“Butterfly populations are bouncy,” said Schultz to The New York Times. “While we think the situation right now is very concerning, we do think there’s a lot of potential to turn it around.”

Monarch Butterfly Population Plummets in California – EcoWatch ›
Western Monarch Population Plummets: Status, Probable … – Frontiers ›
Monarch butterfly ›
Monarch butterfly population critically low on California coast – again ›
monarch butterflies polllinators endangered species climate change pesticides biodiversity

Your Butterfly Photos Could Help Monarch Conservation

imageWestern monarch butterflies spend winter gathered in California’s coastal groves. (Photo by Steve Corey via Wikimedia Commons under CC BY-SA 2.0)


Western monarch butterflies spend winter gathered in California’s coastal groves. (Photo by Steve Corey via Wikimedia Commons under CC BY-SA 2.0)

Millions of monarch butterflies once covered the trees of California’s coastal groves each winter, but now their numbers are dwindling.

The most recent annual Western Monarch Count found that the iconic butterfly population was down 99 percent from populations 40 years ago. Now, researchers and advocates have asked the public to help fill in a missing piece of the monarch migration story by sending them photographs of the butterflies taken before Earth Day on April 22.

“We couldn’t be looking for monarchs right now without the involvement of the community. There just aren’t enough of us,” Washington State University biologist Cheryl Schultz, a lead researcher of the community science initiative, tells Amanda Heidt at the Monterey Herald. “Reaching out to the community means we might be able to learn something where there is virtually no other way to learn it.”

Monarch butterflies are usually thought of as two populations, eastern and western varieties separated by the Rocky Mountains. The populations are very similar in appearance—striking orange with black lines and white spots—but western monarchs are generally smaller and darker in color. The two also follow different migration patterns. Eastern monarchs spend winter in Mexico, while western monarchs congregate on central California coasts to wait out the cold.

In early spring, the monarchs leave their roosts and begin to travel eastward. By the time the butterflies reach the Central Valley, the butterflies breed. A butterfly can lay hundreds of eggs in a few weeks, and the new generation continues the migration. The cycle of generations and migration repeats every few weeks through the summer until the butterflies are dispersed as far as Idaho. And when the weather turns cold and the days get shorter, the insects return to California’s coast.

The overwintering population dropped from about 200,000 in 2017 to less than 29,000 individual butterflies in 2018. By the end of 2019, the count was 29,418—not worse, but not better. The missing information, Schultz says, is the time period when the butterflies leave their winter groves and begin to breed in February, March and April.

“We just don’t know what they’re doing in that middle period and how we can better support the population,” Schultz says to the Monterey Herald. Speaking to Sarah Wright at the Half Moon Bay Review, Schultz added, “Maybe it’s early flowering native milkweeds, or maybe they’re roosting up in the woods, or maybe they need more fuel along the way… Any of those things might help monarchs get from the coastal overwintering sites to breeding sites broadly in the Central Valley.”

To fill that gap, the researchers set up the Western Monarch Mystery Challenge, a ten week program asking the public to submit photographs of monarch butterflies outside of their winter range. It’s okay if the photograph is distant or blurry, they say, as long as submissions by email or iNaturalist mobile app include the date, species, and location. Submissions will be included in the Western Monarch Milkweed Mapper and earn participants an entry in a weekly raffle until the project ends on April 22, according to a statement.

Schultz is optimistic that once experts have a fuller understanding of what the butterflies are up to in early spring, they will be able to identify ways to help. Then, recovery of the historic monarch numbers could be possible.

“Butterfly populations are bouncy,” Schultz tells Karen Weintraub at the New York Times. “While we think the situation right now is very concerning, we do think there’s a lot of potential to turn it around.”

Several states, including California, have ordered residents to stay home except for essential trips to slow the spread of the COVID-19 pandemic. Luckily, people usually don’t have to travel go far to spot a monarch butterfly.

“Most importantly, stay home and stay healthy,” The Western Monarch Mystery Challenge writes on Facebook. “If you happen to sit in the backyard or walk your dog and see a monarch, take a picture and send it to us!”


About Theresa Machemer
Theresa Machemer is a freelance writer based in Washington DC. Her work has also appeared in National Geographic and SciShow. Website: tkmach.com
Read more from this author | Follow @theresakmach

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