UMBS graduate researcher Leslie Decker fears her research may foretell more bad news for monarch butterflies (Danaus plexippus). If her hypothesis is correct, she’ll have evidence that future generations of monarchs will be more susceptible to a malaria-like parasite than they currently are.
“It’s kind of sad” admits Decker, a doctoral student in the University of Michigan’s Department of Ecology and Evolutionary Biology. The monarch population is already facing serious threats, especially from loss of winter habitat and milkweed, which is the exclusive food source for monarch caterpillars.
In addition, monarchs are host to a malaria-like parasite, Ophryocystis elektroscirrha, which harms their ability to reproduce and fly and decreases their overall lifespan. “It appears that rising levels of atmospheric carbon dioxide (CO2) will lower the monarchs’ ability to resist the parasite,” says Decker.
A Beneficial Toxin
Milkweed plants produce toxins called cardenolides. These are what make milkweed sap taste bitter. Because monarch caterpillars eat milkweeds, they taste bad, too. This is the insect’s primary defense against predators.
UMBS researcher and Decker’s advisor, Mark Hunter, has found an additional benefit to monarchs when they consume cardenolides. The chemicals make monarch caterpillars more resistant to the Ophryocystis parasite. The higher the levels of cardenolides in a given plant, the better caterpillars infected with Ophryocystis perform.
In the UMBS pole barn, Decker has mesh cages with pairs of monarch butterflies in them. The butterflies are drinking milkweed nectar – and hopefully mating. “I want to study their grandchildren,” she explains.
It turns out monarch butterflies “know” which plants contain the highest levels of cardenolides. In the wild, infected monarch mothers show a preference for laying their eggs on milkweed plants that are most likely to fill the caterpillars with toxins. “It’s called transgenerational self-medication,” Decker says. Infected mothers create better odds for their offspring by choosing plants with the highest cardenolide levels.
Enter Climate Change
Hunter and his research team discovered that in at least two species of milkweed, the concentration of cardenolides decreased as atmospheric CO2 increased. Atmospheric CO2 concentrations have been rising steadily for years. All widely-accepted climate models predict concentrations will continue to rise for the foreseeable future. Decker wants to find out if monarchs will become more susceptible to Ophryocystis as this change proceeds.
Outside the barn where her monarchs are breeding, Decker is raising their future food. She has covered 40 PVC cubes in clear plastic sheeting. Each cube contains the seedlings of 12 different milkweed species. These include the two species Hunter tested, plus ten others from across North America and Africa. “These are species normally encountered and fed upon by infected monarch butterflies,” she explains.
Decker is pumping additional CO2 into half the cubes. The other half receive ambient air. Once they’re mature, the plants in these cubes will feed Decker’s monarch grandchildren.
By the end of this summer, Decker hopes to have answered two questions. First, are cardenolide levels lower in the plants she raised in an elevated CO2 environment? Second, are the caterpillars that feed on those leaves more easily infected by Ophryocystis?
Between now and then, she will nurture her captive monarch colony and her growing milkweed seedlings. If all goes as planned, her current butterflies’ progeny will themselves lay eggs. Just as those caterpillars are hatching, she will have mature milkweed from her CO2 cages to feed them. Some caterpillars will eat the plants raised in elevated carbon dioxide chambers; others will eat only plants raised in “regular” air. All of them except a control group will be exposed to Ophryocystis.
After those caterpillars pupate and then hatch as butterflies, Decker will be able to collect the ultimate data: how many adults are infected, how virulent their infections are, and to what degree the infection affects their fitness (as measured by longevity).
Decker is not expecting good news for monarchs. But if she can identify milkweed species whose cardenolide levels remain beneficial in the face of rising CO2 levels, those species could be more widely planted to aid the butterflies.
Leslie Decker’s research at the University of Michigan Biological Station is funded in part through a fellowship made possible by the Ann Arbor Branch, Woman’s National Farm and Garden Association.