ANN ARBOR – A recently discovered threat in a new study conducted at the University of Michigan shows that monarch butterflies are facing a possible threat by the rising levels of carbon dioxide in the atmosphere. Milkweed plants that act as hosts for these majestic insects are rapidly losing their medicinal properties due to the increase in atmospheric carbon dioxide.
Monarch caterpillars are solely dependent on these plants for sustenance, as milkweed leaves protect monarchs against predators and parasites by releasing toxins called cardenolides, which give monarchs a bitter taste. The researchers in the U-M Biological Station grew four milkweed species with varying levels of those protective compounds in an experiment.
The experiment only tried to show how rising CO2 levels affected disease susceptibility in monarchs, due to alterations in plant compounds. It involved growing half of the plants under normal carbon dioxide levels, and the other half were exposed to twice as much. These plants were later fed to hundreds of monarch caterpillars.
It was found that the most protective of the four milkweed species, which was grown under increased CO2 levels, lost its medicinal properties which led to a steep drop in the monarch’s ability to ward off even a simple parasite and reduced its lifespan by a week.
We discovered a previously unrecognized, indirect mechanism by which ongoing environmental change – in this case, rising levels of atmospheric CO2 — can act on disease in monarch butterflies. Our results emphasize that global environmental change may influence parasite-host interactions through changes in the medicinal properties of plants. – Leslie Decker, first author of the study
Animals, including humans, use chemicals from plants to fight parasites and diseases. Mark Hunter, U-M ecologist, Decker’s dissertation adviser and co-author of the Ecology Letters paper, 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.” “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 states.
For the fieldwork of the experiment, conducted in 2014 and 2015, they had maintained the current global carbon dioxide concentrations in 20 growth chambers (around 400 parts per million), and the other 20 chambers at the Biological Station were exposed to 760 ppm of CO2, about twice the previous amount. This is the global CO2 level we are expected to reach by the end of the century if the CO2 emissions go unchecked.
The four milkweed species had different levels of the protective compounds. They infected three-day-old caterpillars with controlled doses of a common monarch parasite, Ophryocystis elektroscirrha (a protozoan responsible for shortening of a monarch’s lifespan, and disables their flying as well as reproducing ability) and each caterpillar was fed a single milkweed species.
Unfortunately, our results add to that list and suggest that parasite-infected monarchs will become steadily sicker if atmospheric concentrations of CO2 continue to rise,- Hunter says.
The average lifespan of the individual monarch butterfly in Michigan was recorded to be about a month. The monarchs that fed on Asclepias curassavica showed the steepest decline intolerance, where cardenolide production reduced by almost 25 percent under elevated CO2 levels, and the caterpillars feeding on them showed a decline in parasite tolerance by 77 percent compared to caterpillars that fed on milkweed grown under ambient CO2 levels. A reduction in lifespan of seven days was seen in the former due to parasitic infection, while the latter showed a lifespan reduction by two days.
We’ve been able to show that a medicinal milkweed species loses its protective abilities under elevated carbon dioxide,” Decker said in conclusion. “Our results suggest that rising CO2 will reduce the tolerance of monarch butterflies to their common parasite and will increase parasite virulence. – Leslie Decker
Recent years have seen a rapid decrease in the population of monarch butterflies, and one of the main causes can be traced to their habitat loss due to deforestation in Mexico.