Mounting levels of atmospheric carbon dioxide reduce the medicinal properties of milkweed plants that protect the iconic insects from disease, according to a new study conducted at the University of Michigan (UM), USA, revealing a previously unrecognised threat to monarch butterflies. The milkweed leaves contain bitter toxins that help monarchs ward off predators and parasites, and the plant is the sole food of monarch caterpillars. In a multi-year experiment at the UM Biological Station, researchers grew four milkweed species with varying levels of those protective compounds, which are called cardenolides. The findings of the study refer that half the plants were grown under normal carbon dioxide levels, and half of them were bathed, from dawn to dusk, in nearly twice that amount. Then the plants were fed to hundreds of monarch caterpillars. According to the paper, which was published in the journal Ecology Letters this week, the most protective of the four milkweed species lost its medicinal properties when grown under elevated CO2, resulting in a steep decline in the monarch's ability to tolerate a common parasite, as well as a lifespan reduction of one week. To get to their finding, the researchers of the study looked solely at how elevated carbon dioxide levels alter plant chemistry and how those changes in turn affect interactions between monarchs and their parasites. It did not examine the climate-altering effects of the heat-trapping gas emitted when fossil fuels are burned. “We discovered a previously unrecognised, indirect mechanism, by which ongoing environmental change—in this case, rising levels of atmospheric CO2—can act on disease in monarch butterflies,” said Leslie Decker, first author of the study, adding in the press release of the study “our results emphasise that global environmental change may influence parasite-host interactions through changes in the medicinal properties of plants." Decker is the one who conducted the research for her doctoral dissertation in the UM Department of Ecology and Evolutionary Biology. She is now a postdoctoral researcher at Stanford University. The ecologist Mark Hunter, the Decker's dissertation adviser and co-author of the Ecology Letters paper, said that the findings of the monarch study have broad implications. Many animals, including humans, use chemicals in the environment to help them control parasites and diseases. Aspirin, digitalis, Taxol and many other drugs originally came from plants. “If elevated carbon dioxide reduces the concentration of medicines in plants that monarchs use, it could be changing the concentration of drugs for all animals that self-medicate, including humans,” said Hunter, who has studied monarchs at the UM Biological Station, at the northern tip of Michigan's Lower Peninsula, for more than a decade. “When we play Russian roulette with the concentration of atmospheric gases, we are playing Russian roulette with our ability to find new medicines in nature,” he added.
