When Butterflies Shouldn't Fly Free

Scientists find that releasing captive-bred monarch butterflies can unwittingly spread disease to healthy populations of the imperiled insect.
(Photo: George Rizer/'The Boston Globe' via Getty Images)
Oct 13, 2015· 3 MIN READ
John R. Platt covers the environment, wildlife, and technology and for TakePart, Scientific American, Audubon, and other publications.

Take a look at Twitter or any other social media platform, and you’ll see photo after photo of people excitedly and passionately releasing captive-bred monarch butterflies at weddings, class expeditions, and other public events.

The photos are pretty powerful, and the people obviously think they’re doing something important to help these imperiled insects.

According to scientists, however, these releases may be doing more harm than good.

It turns out that many captive-bred monarchs carry diseases, most notably a monarch-killing parasite called Ophryocystis elektroscirrha, which is closely related to ones that cause malaria and toxoplasmosis. The diseases spread easily at large “monarch farms,” which raise thousands of the butterflies at a time and sell them for release around the country to help their wild cousins.

Monarch populations have plummeted 90 percent over the past two decades, and the United States Fish and Wildlife Service is considering a petition to protect the iconic insect under the federal Endangered Species Act. Scientists have linked the rapid decline of the species to industrialized agriculture in the Midwest, where the widespread planting of genetically modified crops and the skyrocketing use of pesticides have wiped out milkweed, the sole food source for monarch caterpillars.

A letter signed by 10 noted monarch researchers last week said they have observed mass monarch die-offs from the diseases since 2004—which is around the same time that the practice of releasing large numbers of captive-bred monarchs began to become popular.

“Disease can build up in farmed monarchs, which can then spread to wild butterflies,” Sonia Altizer, a professor at the University of Georgia and leading researcher on monarch diseases, said in a statement. “Even monarchs that survive to adulthood can still carry and spread infections, placing already reduced populations of wild monarchs at greater risk.”

Mark Hunter, a professor of ecology and evolutionary biology at the University of Michigan who has been studying monarchs for 20 years, said the problem is similar to diseases that appear on poultry farms and spread through the closely packed population. “You can’t take organisms like monarchs, rear them up in huge densities like this, and not expect to see some kind of evolution,” he said. “The parasite can get more and more and more virulent.”

This increased virulence makes the diseases more dangerous for wild monarchs, Hunter said: “If somebody releases all of these butterflies with these incredibly virulent parasites, they are now available to attract wild insects that have had no recent evolutionary relationship with that virulent parasite.”

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Hunter was not among the scientists who signed last week’s letter, but he coauthored a paper published this week that found that monarchs’ ability to withstand these parasitic infections is closely related to milkweed plants.

Milkweed, it turns out, is more than just the monarchs’ favorite food. It’s also their pharmacy. Previous research has shown that mother butterflies that are infected with the parasites lay their eggs on the most toxic milkweed plants to provide medicine to hatching, hungry larvae. The new research now links that medicinal toxicity to certain naturally occurring fungi in the soil. Plants that grow in soils that contain the fungi become more toxic to the parasites and therefore more beneficial to the butterflies.

That doesn’t mean that every milkweed plant can or should be treated with the fungi to prevent the spread of diseases. Hunter said that would be like giving all human children a dose of penicillin when they are born. “We’re going to get incredible selection for antibiotic-resistance, and suddenly everyone’s going to be in hospital with these multiple resistant bacteria,” he said.

The scientists are not calling for an end to all monarch releases, but they do advise keeping them small and using caution. “If someone really wants to raise monarchs for educational purposes, then we suggest that they collect caterpillars from nearby—such as within the same county—bring them inside to raise them, then let them go when they are adults,” said Sarina Jepsen, endangered species program director for the Xerces Society for Invertebrate Conservation, the organization that issued last week’s letter.

Some educators already do that. Dave Hogg, a professor at the University of Wisconsin–Madison and past president of the Entomological Society of America, said a release conducted in his class last month used butterflies raised from hand-collected eggs and small larvae. He called the parasites that may be present in other commercially raised monarch releases “a great example of unintended consequences of actions for people with good intentions.”

Jepsen and Hunter said raising and releasing monarchs should not be considered a strategy for boosting butterfly populations. “If people want to help monarchs, we recommend that they create, enhance, or restore monarch habitat with native milkweed and other nectar-producing flowers,” Jepsen said. “It's a ‘build it and they will come’ framework.”

Hunter added, “What we really want to encourage the public to do is whenever they have opportunity, find out what’s native to their area, plant those milkweeds, and make sure the soil is in good condition. If we reestablish native plant communities with native fungi, you’ll have that natural pharmacy, and you’ll let the organisms themselves work it out.”