Mounting levels of atmospheric carbon dioxide pose a previously unrecognised threat to monarch butterflies, by reducing the medicinal properties of milkweed plants that protect the iconic insects from disease, a study has found.
Milkweed leaves contain bitter toxins that help monarchs ward off predators and parasites, and the plant is the sole food of monarch caterpillars.
Researchers at the University of Michigan in the US grew four milkweed species with varying levels of those protective compounds, which are called cardenolides.
Half the plants were grown under normal carbon dioxide (CO2)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.
The study showed that 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.
The researchers 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 published in the journal Ecology Letters.
"Our results emphasise that global environmental change may influence parasite-host interactions through changes in the medicinal properties of plants," said Decker, who is now a postdoctoral researcher at Stanford University in the US.
The findings have broad implications, said Mark Hunter, an ecologist at the University of Michigan.
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, he said.
"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.
