Millions of monarch butterflies are killed by parasites yearly

A new study led by Emory University has found that monarch butterflies are increasingly plagued by a debilitating parasite – the protozoan Ophryocystis elektrosirrha (O.E.) – which invades the guts of monarch caterpillars. If the adult butterfly leaves the pupal stage with a severe O.E. infection, it begins oozing fluid from its body and eventually dies. Even the rare butterflies that survive cannot fly well and live shorter than uninfected ones.

The scientists found that the O.E. infection rate has increased from less than one percent of the eastern monarch population in 1968 to as much as ten percent today. 

“We’re seeing a significant change in a wildlife population with a parasitism rate steadily rising from almost non-existent to as high as ten percent,” said study first author Ania Majewska, a postdoctoral fellow in Biology at Emory. “It’s a signal that something is not right in the environment and that we need to pay attention.”

According to Dr. Majewska and her colleagues, this rise in infection rates is caused by increased density of monarchs in places where they lay their eggs. The increased density is due to several factors including the loss of wildlife habitat, the widespread planting of exotic, non-native species of milkweed that butterflies gather in massive numbers to feed upon, and people’s raising large monarch colonies in confined spaces. 

This rise in parasitism may significantly endanger the mass migration of the monarchs – one of the most spectacular displays in the animal kingdom, involving hundreds of millions of western butterflies traveling hundreds of miles down the Pacific Coast to spend the winter in California, and similar numbers of eastern butterflies flying over 3,000 miles from the U.S.- Canadian border to overwinter in Central Mexico. Moreover, their crucial ecological roles as pollinators or essential food for wasps, ants, and other invertebrate predators are also endangered by this worrisome increase in parasitic infection.     

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

“Ultimately, a continuing rise in the monarch’s parasitic infection rate could cause the species to suffer significantly,” added Dr. Majewska. “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, but our findings show how parasites can have a massive impact on wildlife,” concluded Professor de Roode.

The study is published in the Journal of Animal Ecology.

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By Andrei Ionescu, Earth.com Staff Writer