It started with the best of intentions. When evidence emerged that monarch butterflies were losing the milkweed they depend on due to the spread of herbicide-resistant crops in the United States, people across the country took action, planting milkweed in their own gardens. But a new paper shows that well-meaning gardeners might actually be endangering the butterflies’ iconic migration to Mexico. That’s because people have been planting the wrong species of milkweed, thereby increasing the odds of monarchs becoming infected with a crippling parasite.

Habitat loss in both the United States and Mexico has long been the main threat to the North American monarch population. After decades of effort, Mexico curbed deforestation in the butterflies’ winter habitat in the oyamel fir and pine forests of Michoacán and Mexico states. But the loss of milkweed in the United States continues to be a major issue, scientists say. The plant, on which monarchs lay their eggs, used to spring up in between rows of corn, soybeans, and other commercial crops. But today, many farmers plant herbicide-resistant versions of these crops, which allows them to spray their fields with powerful chemicals such as Roundup—killing milkweed in the process. Last year, the number of monarchs that migrated to Mexico was the lowest ever recorded, covering a mere 0.67 hectares of forest, down from a high of 21 hectares in the 1996 to 1997 season. (Scientists in Mexico are planning to announce this season’s count by the end of the month.)

That's why many monarch buffs swung into action. However, the only species of milkweed widely available in the United States is Asclepias curassavica, which is native to the tropics. Tropical milkweed is pretty, easy to grow, and monarchs love it. “If I were a gardener, I would have done the same thing,” says Dara Satterfield, a doctoral student in ecology at the University of Georgia, Athens.

The problem is that tropical milkweed—at least when planted in warm environments like southern Texas and the U.S. Gulf Coast—doesn’t die back in the winter like native milkweed does. When presented with a place to lay their eggs year-round, many monarchs don’t bother making the trip to Mexico at all. Tropical milkweed is “trapping the butterflies” in these new winter breeding sites, says Lincoln Brower, a monarch biologist at Sweet Briar College in Virginia.

But it turns out that year-round tropical milkweed presents an even more direct threat to the butterflies. Milkweed hosts a protozoan parasite called Ophryocystis elektroscirrha (OE). As caterpillars, monarchs ingest the parasite along with their normal milkweed meals, and when they hatch from their chrysalises they are covered in spores. “It’s a debilitating parasite,” Satterfield says. Infected monarchs are much weaker than their healthy counterparts and don’t live nearly as long. In fact, if an OE-infected monarch tries to migrate, it will probably die long before it arrives in central Mexico, Satterfield says.

In that way, the migration is vital to keeping OE under control in the North American monarch population, Satterfield explains. Migrating “weeds out some of the sick monarchs every year,” preventing them from passing the parasite along to their offspring. What’s more, it gives the monarchs a chance to leave behind contaminated milkweed plants, which then die off during the winter. When the butterflies return in the spring “they start over fresh” with new, clean milkweed, Satterfield says. But if the monarchs aren’t migrating, and the tropical milkweed isn’t dying off, OE never goes away.

To figure out if tropical milkweed is increasing OE infections among monarchs, Satterfield enlisted scientists and volunteers to help her sample thousands of butterflies at breeding sites in the United States, as well as in their winter habitat in Mexico. The technique is easy to learn and, with a light touch, harmless: Simply press a small piece of transparent tape against a monarch’s abdomen to collect any OE spores and then send the tape to Satterfield’s lab. She and her colleagues then counted the number of spores trapped by the tape to tally infection rates at different sites.

Monarchs who stayed in the southern United States for the winter were five to nine times more likely to be infected with OE than migrating butterflies were, the team reports online today in the Proceedings of the Royal Society B. In some winter breeding sites, 100% of monarchs they sampled were infected, Satterfield says.

The work proves “absolutely definitively” that tropical milkweed is threatening the monarchs and their migration, Brower says. And the findings are particularly troubling for monarchs returning from Mexico in the spring, he adds. They pass right through these winter breeding sites and could lay eggs on infected milkweed while they are there or mate with infected butterflies. Infecting the returning monarchs with OE “is the last thing we want to do, particularly when the monarchs are in the low numbers that they are now,” Brower says.

Satterfield’s study “quantifies something we knew was a risk,” says Karen Oberhauser, a conservation biologist at the University of Minnesota, Twin Cities. And it’s hitting the monarchs at a particularly vulnerable moment. If the North American population were bigger, the number of winter-breeding, OE-infected butterflies would be trivial compared with the number of hearty monarchs migrating to Mexico. But as the population shrinks, risks like OE can have an outsized effect on overall population numbers, Oberhauser explains. The U.S. Fish and Wildlife Service is now reviewing the monarch’s status under the Endangered Species Act.

There is some good news. Nearly all tropical milkweed in the southern United States is in gardens, Oberhauser says. So if everyone who planted it to help the butterflies can be convinced to replace it with a native milkweed species—or at least cut the plant back every few weeks during the winter—they could quickly put a stop to the destructive winter-breeding trend. (Native milkweed isn’t always as easy to get as tropical milkweed, but it’s starting to become more available online, Satterfield reports.) According to Oberhauser, tropical milkweed is “a problem we can solve.”

*Update, 14 January, 1:13 p.m.: The article originally stated that this season's winter colony count would be announced on 15 January. The announcement has since been rescheduled for a later date.

*Correction, 20 January, 2:28 p.m.: The previous two images for this story depicted the wrong plant. The image for this story has been changed to picture Asclepias curassavica.