Monarch butterflies are vanishing. UGA, Emory study uncovers a new clue.
With their striking markings and epic migrations, few insects inspire as much awe and devotion as the monarch butterfly.
Concern about the iconic species of pollinators has been growing for years though, with studies finding North America’s monarch populations have shrunk dramatically since the 1990s. By 2080, the butterflies could disappear completely from parts of their current range, the U.S. government’s most recent assessment found.
Habitat loss, insecticides and climate change are among the known causes. Still, scientists say there’s much they don’t understand about what’s behind the decline.
Now, a study from University of Georgia and Emory University scientists offers a new explanation, finding that rising temperatures are likely making monarchs more disease-prone.
The findings were published in the December issue of the peer-reviewed scientific journal Ecological Entomology.
The scientists set out to investigate a debilitating and lethal parasite that’s been plaguing monarchs. The culprit is a protozoan called Ophryocystis elektroscirrha, or OE for short.
The single-celled microorganism attacks monarchs when they are caterpillars, causing the butterflies to develop crumpled wings and other deformities during metamorphosis. In severe cases, monarchs infected with OE can be too weak to emerge from their chrysalises after they transform from a caterpillar to a butterfly. If they do make it out, many die soon after.
Even mild cases impede monarchs’ ability to fly, and many that are infected fail to complete their long migrations. Some monarchs have to travel as much as 3,000 miles south to spend the winter in Mexico.
“It’d be like running a marathon with the flu,” said Sonia Altizer, the head of the University of Georgia’s Department of Entomology and a co-lead author of the study.
Not long ago, OE was relatively rare in monarchs.
An earlier Emory study found that about 50 years ago, fewer than 1% of North America’s eastern monarch population was affected by the parasite. Today, 10% or more may be infected.
Altizer said the research team spearheaded by two UGA students — then-Ph.D candidate Isabella Ragonese and undergraduate Christopher Brandon — wanted to know why.
The study found the answer lies, at least in part, in monarchs’ relationship to their preferred food, milkweed.
Monarch caterpillars feed exclusively on plants in the milkweed family, which provide the insects nutrition, as well as protection. The plant’s namesake milky sap contains toxic chemicals that make the monarch butterflies themselves hazardous to predators — and confers some protection against OE.
At a research station in Athens, the scientists raised monarchs with two different species of milkweed — Asclepias incarnata, or swamp milkweed, and Asclepias curassavica, a tropical variety. Half the plants were grown in ambient temperatures. In the others, researchers cranked up the heat in greenhouses, approximating the effects of global warming.
The hotter conditions produced more potent toxins in the milkweed. But the researchers said that surprisingly, the parasitic resistance the insects got from the plants diminished in higher temperatures. At the same time, butterflies raised in the hotter conditions were more likely to be infected and develop more severe disease.
“Something happened where the monarchs just weren’t able to process or benefit from that toxicity when the temperatures were warmer,” Alitzer said. All of it, she said, indicates that “a warmer world might be a sicker world for monarchs.”
Jaret Daniels, curator of the Florida Museum of Natural History’s McGuire Center for Lepidoptera & Biodiversity, said the research provides new insight into how rising temperatures may affect monarchs.
“It’s a complex system to unravel, and it just shows that those interactions between species, host plants and pathogens are really, really important,” said Daniels, who was not affiliated with this study.
As the planet warms from human-caused climate change, Alitzer said there’s still much more to learn about these evolving relationships. The United Nations Intergovernmental Panel on Climate Change estimates global average temperatures are already about 2 degrees higher than they were in the late 19th-century. The planet is expected to get even hotter in the coming decades.
Halting climate change is a global challenge, but Altizer said there are ways individuals can help monarchs thrive.
Homeowners and renters can plant nectar plants, trees and shrubs to create monarch habitat in their yards. Planting milkweed helps, too, but Alitzer said to stick to native varieties. Georgians can also help by reporting monarchs they see to any number of the citizen-led tracking initiatives.
“It’s so important to give scientists a picture of what monarchs are doing and when,” Alitzer said.
A note of disclosure
This coverage is supported by a partnership with Green South Foundation and Journalism Funding Partners. You can learn more and support our climate reporting by donating at AJC.com/donate/climate.
About the Author
Drew Kann is a reporter at The Atlanta Journal-Constitution covering climate change and environmental issues. His passion is for stories that capture how humans are responding to a changing environment. He is a proud graduate of the University of Georgia and Northwestern University, and prior to joining the AJC, he held various roles at CNN.
