How Record-Breaking Heat Waves Add to Risks for Western Monarch Butterflies

About a week ago, several monarch caterpillars were busily munching on the native narrowleaf milkweed I’d planted in my backyard in the San Francisco Bay Area to provide habitat for the imperiled pollinators. Then, a record-breaking fall heat wave triggered warnings about extremely dangerous temperatures across the region.

Hopefully, most of the caterpillars crawled off to find a safe place away from predators to form the pupa, or chrysalis, that envelops them as their wings and adult organs take shape. But at least one fellow wasn’t so lucky. One day, he was active and feeding; the next, he looked lethargic, stopped eating and began to shrivel as his bright yellow stripes disappeared under a sickly dark film. Most likely, he succumbed to a fatal viral or parasitic infection known as black death.

Ecologists have long worried about the effects of a rapidly changing climate on specialists like monarchs, whose reproductive success depends on closely linked interactions with a single family of plants.

Monarch caterpillars feed almost exclusively on milkweed plants within the genus Asclepias, which contain toxic chemicals called cardenolides. The caterpillars absorb the toxins, which deter predators that might otherwise eat them, but typically don’t harm the larvae—that is unless high temperatures cause plants to produce much higher levels of the chemical, as researchers reported in 2018.

Monarchs (and other butterflies) are highly sensitive to temperature, the environmental cue that tells them when to reproduce, migrate and hibernate during the winter. Climate change, particularly warmer falls over several decades, has led to widespread declines of 250 butterfly species, including monarchs, across largely undeveloped landscapes in the West, a 2021 study published in the journal Science found.

Scientists are scrambling to understand how these temperature changes will likely affect monarchs throughout their life cycle, as they develop from egg to butterfly, interact with their sole host plant and navigate the world as adults.

Evidence is emerging, including research published earlier this year, that prolonged exposure to high heat can prove fatal for monarchs.

“Exposure to constant heat of 93 degrees Fahrenheit caused about 50 percent mortality,” said study coauthor Sonia Altizer, an infectious disease ecologist at the Odum School of Ecology at the University of Georgia. Monarch size and survival “declined sharply” at 93 degrees, the researchers found, along with the probability of infection from the monarch’s parasitic nemesis, a single-celled protozoan known as OE, for Ophryocystis elektroscirrha.

Just as the monarch evolved as a milkweed specialist, OE infects only monarchs and a few other butterfly species.

Deaths happened around the time the caterpillars were pupating or trying to emerge as a butterfly, Altizer said.

Interestingly, she added, “We found that the protozoan parasite does not like the heat either.”

It’s conceivable that as rising temperatures in a warming world harm both monarchs and their parasites, lower infection rates could potentially offset the negative impacts of extreme heat on monarchs. But with so many other variables at play—differences in monarch immunity, parasite strains’ heat tolerance, milkweed quality and chemistry, to name a few—more research is needed to tease apart all the possible scenarios.

In a three-year field experiment that recreated elements of the natural landscape in California’s Central Valley, researchers found that both monarch eggs and caterpillars experienced strong negative effects from heat stress late in the season. “We speculate that the increasing incidence and intensity of heat waves may have reduced the developmental success of monarchs,” the authors concluded in the study, published in 2022.

Monarchs can tolerate short periods of heat as caterpillars, as long as they find respite each day, research shows. Altizer thought heat stress may have played a role in the rapid decline of the caterpillar that succumbed to disease in my garden.

Conservation Conundrum

Biologists gauge the health of monarch butterfly populations by tallying their numbers over the winter when they take a break from breeding and cluster in groves of trees. North America has two populations of migratory monarchs: the western monarchs, which breed west of the Rocky Mountains, and the much larger eastern population, which overwinters in Mexico. They’re genetically similar and have both suffered major declines over the past several decades.

Scientists have identified loss of milkweed habitat and use of weed-killing pesticides across the eastern monarchs’ core breeding ground in the Midwest as a major driver of the population’s decline. Loss of overwintering habitat appears to be a primary cause of the western monarchs’ decline, though in both cases, experts warn, it’s likely that many factors are playing a role.

“Land-use change, including agricultural practices, chemical inputs, habitat fragmentation, pollution, development, disturbance, and so forth, is what has been incrementally creeping up over the last century,” wrote Anurag Agrawal, Cornell University evolutionary ecologist and author of “Monarchs and Milkweed,” in a 2019 commentary in the journal PNAS. “Which of the many aspects of this long-term environmental crumbling is responsible for monarch declines is unclear.”

Western monarchs overwinter in groves of non-native eucalyptus and native Monterey pine and cypress trees, primarily at sites along the Pacific coast. Last year, the nonprofit Xerces Society for Invertebrate Conservation, which supervises the Western Monarch Count, reported 233,394 butterflies at 257 overwintering sites along the coast of California, northern Baja California and Mexico, as well as a few inland sites. California’s Central Coast hosts the largest overwintering sites, where butterfly lovers can see as many as 10,000 in Santa Cruz and more than 15,000 in Santa Barbara.

The count offered a bit of good news after a dire few years. A 2017 study reported a “sharp decline” in the western population, from an estimated 4.5 million in the 1980s to about 300,000 in 2016. More losses followed.

By the winter of 2018-2019, overwintering western monarch populations had plummeted to about 30,000, researchers reported in 2019. The decline followed a prolonged multi-year drought, at the time the driest consecutive four years on record, a late rainy season marked by heavy storms and a summer filled with wildfire smoke. Unusually heavy rains during the spring of 2018 may have played a role in the population decline, the researchers noted, and it’s possible the storms compounded problems with habitat quality at overwintering sites or inland breeding locations.

As if those declines weren’t bad enough, “the bottom fell out” in 2020, with only 2,000 butterflies overwintering, said Emma Pelton, senior conservation biologist with the Xerces Society who contributed to the 2017 and 2019 studies.

The overwintering population of western monarchs currently hovers around 5 percent of historic figures.

Climate change has made the butterfly’s range warmer, drier and more prone to heatwaves, wildfires and winter storms. And groves at the Central Coast wintering sites have still not recovered from the historic drought during the last decade, said Pelton. The eucalyptus trees that monarchs rely on were “really hammered” by drought and created a fire risk, she said.

Such indirect effects of drought and heat stress on critical overwintering habitat are an unappreciated impact of climate change, she said.

That same winter, when just a few thousand butterflies showed up to hibernate, scientists reported “substantial numbers” of monarchs breeding primarily on non-native ornamental milkweed at two urban sites about an hour south of San Francisco. Non-native tropical milkweed species, unlike western natives, stay green year-round and can act as a cue to fall migrants—which emerge in a nonreproductive state called diapause—that it’s breeding time.

However, year-round milkweed accumulates higher concentrations of that parasitic nemesis OE. Monarchs that stay on these patches to breed during winter have higher parasite loads and become a vehicle for disease transmission. OE infected as many as 77 percent of newly emerged butterflies throughout the study, published in 2021.

Western monarchs were found breeding in winter before the 2021 study. Scientists in Altizer’s lab sampled more than 1,200 monarchs at 42 year-round breeding sites in Southern California between 2013 and 2016. Most of the year-round breeders were on non-native tropical milkweed (A. curassavica), a popular garden plant, though a few sites had native species that supported breeding until they died back in late winter.

The prevalence of OE infection was nine times higher among monarchs sampled in gardens with year-round breeding compared to monarchs that had migrated to overwintering sites during that time, they reported in the study, published in 2016.

Can Adaptation Save Monarchs?

David James, who documented the winter breeders, supports efforts to grow native milkweed where possible and grows it in his own garden in western Washington. However, he believes that winter breeding on non-native milkweed may reflect the species’ ability to adapt to rapidly changing conditions and could be important for the continued sustainability of western monarch populations.

“Any milkweed is good milkweed as far as the monarch is concerned,” he said.

It is not a position that is widely shared among monarch ecologists and conservation biologists.

There’s been some debate about whether monarchs, which are found in more than 90 countries and islands around the world, evolved as a tropical species. However, a genetic analysis of the monarch’s evolutionary history and dispersal, published in 2014, found that it originated as a migratory species in North America, which suggests millennia of munching milkweed species native to that environment.

That means non-native milkweeds could lead to a host of unforeseen effects on monarchs, experts say.

“There’s a healthy debate going on in the research world around the role of tropical milkweed,” said Pelton. The evidence is strong that it adds to disease, she said, so the question is whether the increase in butterflies counterbalances the increase in disease.

Pelton, along with her Xerces colleagues, takes a precautionary approach. “We have a struggling population, and this is a non-native plant with some proven evidence that it increases OE,” she said, adding that it may also interfere with migration.

OE is an infection that monarchs evolved with, Pelton said, which is probably why they migrate. Infected individuals may not be strong enough to travel and could die on the way, reducing the number of disease carriers, while migration reduces population density and, thus, transmission.

“Throwing more disease in the mix with these declining populations just seems like a bad idea,” Pelton said. “All things being equal, let’s use the native plant the monarch co-evolved with, that’s providing resources to other animals and not adding this X factor, especially in a changing climate.”

Pelton is sympathetic to gardeners who enjoy seeing caterpillars on their tropical milkweed and believes it’s important not to demonize people for wanting to grow it. But there’s some dissonance between the personal perspective—the thrill of seeing butterflies in your yard—and the bigger perspective of what’s actually aiding western monarchs over the long term, she said.

“What they need is enough habitat that’s not poisoned by pesticides or impacted by extreme heat,” she said.

“Saving an iconic butterfly is important and would help us sustain beauty, wonder, and majesty in nature,” Cornell’s Agrawal said in his PNAS commentary. But he sees the monarch as a sentinel species that offers warnings about the environmental health of the continent and biodiversity at large.

Humans are very good at tinkering, Agrawal told Inside Climate News. “When we have tinkered in known ways, there have been unintended consequences. As we are tinkering in unintentional ways,” he said, citing climate change as an example, “there are similarly unknown consequences.”

While monarchs are experiencing higher levels of disease, changing migratory patterns and habitat loss, Agrawal said, they’re also flexible, and he believes the species will persist.

Still, he said, “We should do less tinkering and more habitat preservation.” And that means protecting and restoring the wildlands and native species that have supported monarchs over millennia.

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