Toxic Tipping Point

The open top chambers used to warm plots containing milkweed plants and monarch caterpillars

Matt Faldyn

Monarch butterflies (Danaus plexippus) fend off predators and parasites by ingesting cardenolide, a toxic steroidal compound produced in milkweeds, genus Asclepius, on which monarchs feed exclusively. The amount of cardenolide produced varies by Asclepius species. Higher amounts provide more protection—up to a point. New research shows that increased temperatures increase cardenolide, but in one of the two milkweed species studied, concentrations reached levels that were detrimental to monarchs.

Researchers led by graduate student Matthew Faldyn and biologist Bret Elderd, at Louisiana State University in Baton Rouge, tested the effects of temperature on potted milkweed plants and on the monarch larvae that fed on them. They placed eighty first-instar monarchs on forty potted milkweed plants, of which half were a native milkweed species, A. incarnata, and half were tropical milkweed, A. curassavica. Plants of each species were then further divided: half were kept in open-top containers that increased inside, daytime temperature by about three degrees Celsius, and the other half were kept uncovered. The monarch larvae were put in butterfly bags and placed around the plants for two weeks to feed until pupation.

Cardenolide concentrations were found to be thirteen times higher in the tropical milkweed plants than in the native plants. Tropical milkweed plants also had higher levels of nitrogen in their leaves compared to the native plants. These factors combined to allow for higher survival of the larvae on the tropical milkweed than on the native milkweed—but only under ambient temperature conditions. 

A monarch butterfly feeding in the Elderd Lab at Louisiana State University, Baton Rouge, Louisiana

Matt Faldyn
At higher temperatures, cardenolide concentrations in the tropical milkweed plants increased by about 50 percent compared to their counterparts at ambient temperatures. This decreased larvae survival rates by roughly 75 percent. The warmer conditions in the tropical plants also decreased the average weight of adults from 0.5 grams to 0.4 grams. Temperature did not significantly affect weight or survival of caterpillars raised on the native milkweed. Unfortunately, many monarchs in the southern United States are using tropical milkweed as a host plant and are not overwintering in Mexico. As toxicity rises with increased temperatures, even from temporary heat waves, monarchs could be in an ecological trap. Consequently, researchers suggest that efforts to increase monarch populations should focus on planting native milkweed species. (Ecology)