Milkweed grown in carbon dioxide-filled chambers could help scientists predict the fate of monarch butterflies.
The monarch butterfly population is already on the decline, and milkweed is their primary food source. The milkweed used in the experiment was exposed to the CO2 levels expected to exist in over a century if greenhouse gas emissions continue to rise from the burning of fossil fuel, the University of Michigan reported. The milkweed was fed to hundreds of hungry monarch butterflies and caterpillars this summer.
Past research has shown some species of milkweed produces lower levels of protective cardenolide toxins that also help monarchs fight disease and parasites when exposed to elevated levels of carbon dioxide.
"When I heard that, it really set off an alarm," said U-M doctoral student Leslie Decker. "If toxins that are very active against parasites are decreasing under elevated CO2, what does that mean for the susceptibility of monarchs in the future?"
To answer this question, four species of milkweed were grown in PVC-framed Biological Station growth chambers. Half of the plants were grown under normal CO2 conditions (400 parts per million) while the others were exposed to about twice as much.
The researchers measured the cardenolide toxins present in each plant so they knew how high of a dose each caterpillar received. Three-day-old caterpillars were the exposed to controlled doses of the common monarch parasite Ophryocystis elektroscirrha.
"We know exactly how many spores each caterpillar ate, as well as the exact dose of the milkweed toxin they received," said laboratory of ecologist Mark Hunter.
The team hand raised 480 adult monarch butterflies from eggs under these varied conditions, and recorded their lifespans as well as the number of spores on the outside of every butterfly carcass.This is the second season of the long-term study, and there are four more to go. If the final findings reveal certain species of milkweed help monarchs fight off parasites, it could be possible to create a management plan that involves planting beneficial species of milkweed.
"This is a way determine how elevated CO2 changes the chemistry of these plants and how those changes work their way up the food chain," Decker said. "At the same time, a lot of people reared monarch butterflies as children and still have a sentimental attachment to them. So our work may motivate some people to care more about the ways global change is altering our world."