Monarch butterflies increasingly plagued by parasites, study shows

Carol Clark,; Beth Gavrilles,

Contact: Ania Majewska,; Sonia Altizer,;

North American monarch butterflies, like those shown here overwintering in Mexico, are increasignly plagued by a debilitating parasite, with major implications for their conservation. Photo: Sonia Altizer.

North American monarch butterflies are increasingly plagued by a debilitating parasite, according to a new analysis from Emory University and the University of Georgia.

The findings, recently published in the Journal of Animal Ecology, have major implications for the conservation of these iconic insects and their annual migration.

“We’re seeing a significant change in a wildlife population with a parasitism rate steadily rising from almost non-existent to as high as 10 percent,” said lead author Ania Majewska, a FIRST post-doctoral fellow in Emory’s Department of Biology who began researching monarch butterflies as a doctoral student in Ecology at the University of Georgia. “It’s a signal that something is not right in the environment and that we need to pay attention.”

The analysis drew from 50 years of data on the infection rate of wild monarch butterflies by the protozoan Ophryocystis elektrosirrha, or O.E. The results showed that the O.E. infection rate increased from less than one percent of the eastern North American monarch population in 1968 to as much as 10 percent today.

The O.E. parasite invades the gut of the monarch caterpillars when they eat dormant spores on milkweed leaves. If the adult butterfly leaves the pupal stage with a severe parasitic infection, it begins oozing fluids from its body and dies. Even if the butterflies survive, as in case of a lighter infection, they do not fly well or live as long as uninfected ones.

“Trying to complete this perilous migration while infected with this parasite would be like trying to complete the Boston Marathon with the flu,” said co-author Andy Davis, an assistant research scientist in the University of Georgia Odum School of Ecology. “Sick butterflies have a hard time reaching the finish line.”

The rise in parasitism, the researchers warn, may endanger the mass migration of the monarchs, one of the most spectacular displays in the animal kingdom, involving hundreds of millions of butterflies. Each fall, the western monarch population flies hundreds of miles down the Pacific Coast to spend the winter in California. Meanwhile, on the other side of the Rocky Mountains, eastern monarchs fly from as far north as the U.S.-Canadian border to overwinter in Central Mexico, covering as much as 3,000 miles.

“Our findings suggest that tens of millions of eastern monarch butterflies are getting sick and dying each year from these parasites,” said Jaap de Roode, Emory professor of biology and senior author of the study. “If the infection rates keep going up, fewer and fewer monarchs will be able to survive to migrate to their overwintering sites.”

One contributor to the rise in the parasitism rate is the increased density of monarchs, the study found. The researchers posit that this may be due to many factors, including the loss of wildlife habitat; the widespread planting of exotic, non-native species of milkweed; and people raising monarchs in large numbers in confined spaces.

Previous research led by study coauthor Sonia Altizer, the Georgia Athletic Association Professor of Ecology at the Odum School, implicated the widespread planting of tropical milkweed, a species readily available to gardeners, as an important contributing factor to rising O.E. infections. Because tropical milkweed can bloom year-round in the southeastern U.S., the monarchs they attract can forego their annual migration to Mexico, keeping them in increasingly O.E.-contaminated habitat.

For the current paper, the researchers wanted to investigate the parasite’s infection rate in monarch populations over time. They used data sets stretching back five decades and at a continent-wide scale, from as far north as Canada to central Mexico, and from the eastern to western seaboards of North America.

“To have this many samples—almost 60,000 butterflies—across such a large time and space, gives us an amazing window into the history of infection for North American monarchs,” said Altizer. “The vast data set allowed us to look at trends over time, differences between populations, seasonal changes that coincide with the annual migratory cycle, and how infection tracks changes in monarch density and environmental conditions.”

Among the factors that may be contributing to the increased monarch density associated with the rise in parasitism, the researchers noted, is patchier or more concentrated milkweed growth across the landscape. If female monarchs must cluster more densely to lay their eggs, the butterfly “nurseries” become more crowded with caterpillars.

“One thing that the COVID-19 pandemic taught us is that social distancing can help reduce the spread of an infectious disease,” De Roode said. “The same holds true for monarchs and the O.E. parasite.”

The researchers further hypothesized that people raising monarch caterpillars in large numbers—trying to boost monarch conservation or for commercial purposes—may be keeping them in crowded conditions that foster the spread of the parasite.

“Ultimately, a continuing rise in the monarch’s parasitic infection rate could cause the species to suffer significantly,” Majewska said. “If tens of millions of them are dying annually from parasitic infections, then an extreme weather event during the winter in Mexico might reduce the population to a level that could be dangerous for their genetic diversity.”

“Parasitism is often overlooked in conservation efforts,” de Roode added, “but our findings show how parasites can have a massive impact on wildlife.”

The research was supported by the National Institutes of Health (NIGMS K12 Postdoctoral Fellowship at Emory University, award 5K12GM000680-19), the National Science Foundation, and the Strategic Environmental Research and Development Program of the Department of Defense, Environmental Protection Agency and Department of Energy.