Heat and CO2 scenarios suggest a milkweed species becomes toxic or useless to caterpillars
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Climate change could make a showy invasive milkweed called a bloodflower even more of a menace for monarch butterflies than it already is.
Monarch caterpillars, which feed on plants in the milkweed family, readily feast on Asclepias curassavica. Gardeners in the southern United States plant it for its showy orange blooms, yet the species “is turning out to be a bit of a nightmare,” says Mark Hunter of the University of Michigan in Ann Arbor.
Monarchs (Danaus plexippus) migrating south to Mexico in the fall come across bloodflower bonanzas and don’t bother to keep on flying. Full migration normally prevents a harmful Ophryocistis parasite from building up in the insect population. Cutting the cycle short lets infection flourish.
In experiments, bloodflowers grown in outdoor enclosures under high carbon dioxide concentrations, around 760 parts per million, don’t make as much medicinal cardenolide as normal, Hunter and colleagues report July 9 in Ecology Letters. Caterpillars need these compounds to help fight parasites. Levels of two particularly potent forms of cardenolide stayed low. Parasites were more damaging to caterpillars chewing through these futuristic flowers than to those caterpillars fattening on plants grown under current atmospheric conditions.
Higher temperatures due to climate change, however, may boost cardenolides instead of reducing them, Hunter and different colleagues reported in the May Ecology. That could turn the bloodflower species toxic to monarchs, according to a test growing milkweeds in enclosures with daytime temperatures raised some 3 degrees Celsius higher than outside air. A native milkweed, A. incarnata, didn’t get close to toxic.
Researchers don’t know how the opposing effects of CO2 and heat might act on cardenolides overall. Regardless of how further research on that question turns out, bloodflowers are already a threat to monarchs. Hunter urges gardeners who can’t resist growing the plants to at least cut them back in the fall, so that they won’t derail the monarch migration.
Editor's Note: This story was updated on July 11, 2018 to replace the top image with a picture of a monarch, not a queen butterfly.
L.E. Decker et al. Elevated atmospheric concentrations of carbon dioxide reduce monarch tolerance and increase parasite virulence by altering the medicinal properties of milkweeds. Ecology Letters. Published online July 9, 2018. doi:10.1111/ele13101.
M.J. Faldyn, M.D. Hunter and B.D. Elderd. Climate change and an invasive, tropical milkweed: an ecological trap for monarch butterflies. Ecology. Vol. 99, May 2018, p. 1031. doi:10.1002/ecy.2198.
D.A. Satterfield, J.C. Maerz and S. Altizer. Loss of migratory behavior increases infection risk for a butterfly host. Proceedings of the Royal Society B. Vol. 282, February 22, 2015. doi:10.1098/rspb.2014.1734.
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