Monarch butterflies use medicinal plants to treat offspring

ANN ARBOR, Mich.—Monarch butterflies appear to use medicinal plants to treat their offspring for disease, research by biologists at the University of Michigan and Emory University shows.

"We have shown that some species of milkweed—the larva’s food plants—can reduce parasite infection in the monarchs," said Jaap de Roode, the Emory evolutionary biologist who led the study. "And we have also found that infected female butterflies prefer to lay their eggs on plants that will make their offspring less sick, suggesting that monarchs have evolved the ability to medicate their offspring."

Few studies have been done on self-medication by animals, but some scientists have theorized that the practice may be more widespread than we realize.

"Several criteria must be met in order to demonstrate that self-medication actually is occurring," said U-M chemical ecologist Mark Hunter, who collaborated with de Roode’s group on the research. "In this study, all of those criteria were met, making it one of the first clear demonstrations of self-medication in an animal. In addition, it’s the first example of trans-generational medication, with the mother’s behavior benefiting her offspring."

Their findings were published online Oct. 6 in the journal Ecology Letters. The findings also may have implications for human health, said Hunter, who is the Henry A. Gleason Collegiate Professor in Ecology and Evolutionary Biology.

"When I walk around outside, I think of the plants I see as a great, green pharmacy. But what also strikes me is how little we actually know about what that pharmacy has to offer," he said. "Studying organisms engaged in self-medication gives us a clue as to what compounds might be worth investigating for their potential as human medicines. Researchers have studied the kinds of leaves that primates eat in forests, but this work with butterflies stresses the point that even insects in our own back yard can be useful indicators of what might be medicinally active."

Monarch butterflies are known for their spectacular migration from the United States to Mexico each year, and for the striking pattern of orange, black and white on their wings. That bright coloration is a warning sign to birds and other predators that the butterfly may be poisonous.

Monarch caterpillars feed on any of dozens of species of milkweed plants, including some species that contain high levels of heart poisons called cardenolides. These chemicals do not harm the caterpillars, but make them toxic to predators even after they emerge as adults from their chrysalises.

Previous research has focused on whether the butterflies choose more toxic species of milkweed to ward off predators. The researchers wondered if the choice could be related to the protozoan parasite, Ophryocystis elektroscirrha, that infects monarchs. The parasites invade the gut of the caterpillars and then persist when the caterpillars become adult monarchs. An infected female passes on the parasites when she lays her eggs. 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, they do not fly as well or live as long as uninfected ones.

Experiments in de Roode’s lab showed that a female infected with the parasites prefers to lay her eggs on a toxic species of milkweed, rather than a non-toxic species. Uninfected female monarchs, however, show no preference. The research also showed that while adult females and caterpillars are incapable of medicating themselves, the adult female’s plant choice protects her offspring from parasites.

"This result shows that organisms are able to make decisions about what they eat, but also about what their offspring eat, and that the decision is based on medicinal content," said Hunter, whose interest in insect feeding choices dates back to his days as a postdoctoral fellow Penn State, where he was part of a group studying gypsy moths.

Hunter and de Roode recently received funding from the National Science Foundation, which they will use to see if the lab results can be replicated in nature, in different populations of monarchs in various regions of the world, and to identify the chemicals that account for the medicinal properties of the milkweed plants.

In addition to Hunter and de Roode, the paper’s authors are Thierry Lefèvre and Lindsay Oliver of Emory.