Bloodflowers’ risk to monarchs could multiply as climate changes

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.