Monarchs Use Milkweed as Preventive Medicine—but Climate Change May Wreck the Pharmacy

Most people know that monarch butterflies can’t exist without milkweed. As caterpillars, the monarchs feed on milkweed plants exclusively, absorbing the milkweed’s poisons in order to ward off birds and other predators. On their epic migration across the North American continent, the butterflies also lay their eggs on these plants, relying on the noxious taste of the leaves to keep their brood safe from grazers while simultaneously providing a buffet for the next generation when it hatches.

But milkweed plays another role in the lives of monarchs that goes largely overlooked. Some of the compounds produced in the plants’ leaves act like medicine for the butterflies, both protecting them against parasites, like the protozoan Ophryocystis elektroscirrha, and helping them cope with parasitic infections when they do occur.

Unfortunately, a new study published in the journal Ecology Letters finds that as carbon dioxide levels rise in the atmosphere, the most medicinal of those milkweed plants start to lose their juju. “Our study shows that a loss of medicinal protection caused by elevated CO₂ makes the parasite more virulent,” says lead author Leslie Decker, an ecologist now at Stanford University. (The research was part of Decker's doctoral dissertation at the University of Michigan.)

In work conducted between May 2014 and May 2015, the researchers raised various species of milkweed under controlled conditions. Some of the plants grew with present-day concentrations of CO₂, and some with levels like those climate scientists expect in the next 100 to 150 years. They then let monarch caterpillars go to town on these plants to see if the CO₂ levels would change the way the plants helped the insects fight off infection.

The scientists found that under the futuristic CO₂ conditions, all milkweed species showed dips in their concentrations of cardenolides, the steroid cocktails they produce for self-defense. But one species stood out. Monarch caterpillars that fed on Asclepias curassavica, or tropical milkweed, lost 77 percent of their tolerance to protozoan parasites. Without the normal dose of their milkweed medicine, the monarchs died a full week earlier than those that ingested tropical milkweed grown at today’s levels of CO₂. This is concerning, because of the species tested, tropical milkweed proved to be far and away the most medicinal of all the plants under the current CO₂ regime.

“Rising CO₂ levels are likely to disrupt our natural systems in highly complex ways, including by unraveling long-established host/parasite relationships,” says Sylvia Fallon, director of NRDC’s Wildlife Conservation Project. “This would pose yet another grave threat for a species like the monarch butterfly, whose populations have already plummeted due to habitat loss and pesticide use.”

The findings are scary, especially considering that the monarchs and their massive North American migration are already under attack from all sides. Pesticides threaten the insects’ summer breeding grounds in the United States and Canada, and illegal logging is chipping away at their winter homes in the mountains of Mexico. Agriculture has already eradicated milkweed across huge swaths of the American Midwest. And even the milkweed plants available at the big box stores, which people buy to help the butterflies, can sometimes be laced with systemic poisons.

The good news is that the fate of the monarchs is far from sealed. While the new study finds that heightened CO₂ levels make monarchs more vulnerable to infections, this shift in favor of the parasites may be only temporary. You see, it doesn’t help the parasites to kill their hosts; living inside adult butterflies is the only way these parasites can proliferate. Therefore, Decker says, the protozoan “evolves to only harm the monarch host as much as it has to in order to produce an optimum number of spores,” to make sure it can pass on its genes.

Right now, monarchs are able to temper the negative effects of the parasite by eating milkweed, but if that strategy slowly loses its value, the whole interspecies relationship may end up being reshuffled. Why? Because more virulent parasite strains will kill their hosts too quickly to spread their own spores, and then natural selection will likely weed them out, leaving only the parasitic strains that don’t harm the monarchs enough to kill them.

Think of it like Goldilocks—only the bears are butterflies, the porridge they eat is milkweed of varying medicinal qualities, and the little girl is an obligate neogregarine parasite that everyone agrees is trespassing.

Unfortunately, even if natural selection makes these parasites less virulent over time, the change in the compounds produced by the milkweed will also make monarchs less able to cope with the ones that do survive. “The harm inflicted by each parasite spore will increase,” Decker says, “spelling out bad news for the monarch.”

The impact of the high-carbon scenarios tested by Decker and her team highlights the importance of protecting these insects right now, if only to give them a buffer to survive what may lie ahead. “We must do everything we can to build a more stable and resilient population by planting more pollinator habitat and reducing the use of agricultural pesticides,” says Fallon. If not, one of the world’s most impressive journeys may come to an end.

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