Rising Carbon Dioxide Levels Endanger Monarch Butterflies' Main Source of Sustenance

A new study finds high CO2 levels reduce the medicinal properties of milkweed plants that protect monarch butterflies from disease.
Loukia Papadopoulos
The photo credit line may appear like thisAustin Thomason/University of Michigan

Monarch butterflies have been facing quasi-extinction in recent years due to habitat loss and the disappearance of milkweeds, the host plant of the insects whose consumption also protects them from parasites and diseases. Now, a new study conducted at the University of Michigan has uncovered a new environmental threat to the survival of this colorful species.

The research reveals that the rise of atmospheric carbon dioxide has resulted in a decrease of the traditional medicinal properties of milkweed plants. Consumption of the deficient plants results in weakened insects ill-equipped to fight off parasites and illness.

A nefarious indirect mechanism

“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,” said Leslie Decker, first author of the study.

The researchers grew half a batch of milkweed plants in normal carbon dioxide levels, around 400 parts per million, and another half in twice that amount, 760 ppm of CO2. The plants were then fed to monarch caterpillars and the insects studied for resulting effects.

The scientists discovered that even the most protective of the four milkweeds studied had lost many of its medicinal properties when exposed to high CO2 levels.

“We’ve been able to show that a medicinal milkweed species loses its protective abilities under elevated carbon dioxide. Our results suggest that rising CO2 will reduce the tolerance of monarch butterflies to their common parasite and will increase parasite virulence.” said Decker, who conducted the research for her Phd in Ecology and Evolutionary Biology.

In April of 2018, the Mauna Loa Observatory in Hawaii revealed carbon dioxide concentrations in the atmosphere exceeded an average of 410 parts per million across the entire month. Environmentalists and scientists have warned since 2016 that the 400 parts per million threshold that was reached means our planet will never return to measurements below it again. 

Playing Russian roulette with medication

This rise of CO2 and its effects on milkweeds is not only concerning for butterflies. Chemicals in plants are used by many species, including humans, in warding off parasites and fighting diseases either naturally or through the production of plant-based drugs such as Aspirin.


“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,” said University of Michigan ecologist Mark Hunter, Decker’s dissertation adviser and co-author.

The study, supported by the National Science Foundation, was also co-authored with Jacobus de Roode of Emory University. It is scheduled for publication on July 10 in the journal Ecology Letters.


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