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How some well-intentioned Americans trying to save monarch butterflies may actually be hurting them

This timelapse from the U.S. Fish and Wildlife Service shows a monarch butterfly emerging from a chrysalis. (Video: U.S. Fish and Wildlife Service via YouTube)

Updated Jan. 27

People love the monarch butterfly, with its beautiful orange and black wings and its endlessly intriguing migratory journey. But some may be loving it to death.

That’s the tragic essence of a new study, which suggests the efforts of many backyard gardeners to save the creatures may in fact be unintentionally contributing to their endangerment.

The North American monarch butterfly is already in such a state of decline that it’s up for consideration as an endangered species thanks largely to habitat loss blamed on use of agricultural herbicides. Concern for the monarch is so widespread that for some years now, millions of American gardeners have engaged in their own crusade to support the monarch population by buying and planting milkweed, on which the monarch larvae relies for sustenance.

They mean only the best for the monarch. But the result, in some instances, is the exact opposite. Indeed, the new study confirms what many monarch researchers have long feared: Some of these efforts may actually be sabotaging the monarch, rather than rescuing it. And in the process, some saviors of the monarch are now becoming an additional threat.

Why? The milkweed some of these well-intentioned monarch lovers have been planting is the wrong kind, and according to the study, may be turning migratory creatures into sedentary ones. That makes them unhealthy, and more susceptible to the spread of a crippling parasite that can only hasten the species endangerment.

Nobody’s blaming anyone. “If I were a gardener, I would have done the same thing,” Dara A. Satterfield, one of the study’s authors, told Science Magazine. But the problem is nonetheless real and now inescapable.

There’s no doubt, according to monarch researchers, that there’s been a massive decline in the milkweed essential to the survival of the monarch. The cause: Herbicides used in agriculture that, while great for corn and other crops, have killed off untold acreage of milkweed along the migratory routes used by the North American monarch to make their way from chilly northern climes to a small patch of mountainous forest in Mexico.

When news of this problem began to spread a few years ago, largely through the efforts of conservation organizations, gardeners in the southern U.S. started going out and getting their own milkweed.

Here’s the problem, according to the study published in the Proceedings of the Royal Society by Satterfield, John C. Maerz and Sonia Altizer, all of the University of Georgia. The type of milkweed on which the monarchs rely naturally is a type that deteriorates starting in the fall months, thus providing an incentive for the butterflies to keep moving along their way south.

The type most available commercially — and purchased by the backyard conservationists — is known as a tropical milkweed, which “continues to produce new foliage and flowers during the autumn and winter,” the study says. It encourages the monarchs to settle in the southern U.S. rather than continue their journey.

In other words, these monarchs, like people, are getting sedentary. And sedentary butterflies, like sedentary Americans, are vulnerable.

The researchers conducted tests on some of these sedentary butterflies and found that “diminished migration increases infection risk” among them, specifically infection with a protozoan parasite called Ophryocystis elektroscirrha (OE), which does the most horrible things. Adult monarchs infected with OE wind up covered with spores, which they scatter onto eggs or the milkweed, to be ingested by the larvae while feeding.

The result, say the researchers, are “wing deformities, smaller body size, reduced flight performance and shorter adult lifespan.” In the natural world of survival of the fittest, these butterflies would be unfit, unable to migrate or reproduce. The weakest would be culled.

In the artificial world where they are fed by humans, that doesn’t happen.

As Science Magazine explained it:

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.

“The widespread decline of migratory monarchs in North America has been widely publicized,” the authors write, “with most attention focused on habitat loss as a major cause. Shifts towards year-round breeding on tropical milkweed, resulting in high rates of OE infection, could pose an additional emerging threat to the long-term viability of migratory monarchs.”

“Our results,” they report, “add to a growing number of studies that show migratory species worldwide are shifting the timing and spatial patterns of movement in response to human activities.”

Update: Morning Mix has heard from a number of readers concerned that the way the study was described in this article, particularly in the headline, overstated the tropical milkweed problem and could discourage people from planting milkweed in aid of the monarch butterfly. They have a valid point and we have changed the headline to reflect it.  Here’s some excellent perspective from Sylvia Fallon, senior scientist at the Natural Resources Defense Council, posted on the NRDC blog, Switchboard and quoted at length with permission:

Recently there have been a number of articles in the press about how people are planting the wrong kind of milkweed and that it’s actually harming monarch butterflies. Inflammatory headlines have warned about how well intentioned people may actually be “destroying” monarchs and how “the plan to save monarchs” by planting milkweed has “backfired.” Well, I’m here to tell you that there is no need to freak out. And Please, KEEP PLANTING MILKWEED! Here’s why.
A recent study showed that one particular variety of milkweed, an exotic plant called tropical milkweed, is posing some problems to monarchs in certain areas–particularly in the southern US where conditions allow tropical milkweed to survive year round. (The 100+ other native varieties of milkweed typically die back in the winter). In coastal areas of the southern U.S., tropical milkweed that grows year-round can allow some monarchs to breed all year and skip their migration to Mexico where they overwinter. Year round use of milkweed also increases disease transmission among the butterflies. For these reasons, the researchers have warned that tropical milkweed in the southern US may be causing harm to some monarchs–although they are not sure to what extent this might be affecting the overall monarch population.
Here’s what you need to know about the broader implications of this study. The problem posed by tropical milkweed is restricted to a limited part of the monarch’s range–namely portions of the southern US. Furthermore, it can be managed by having gardeners cut back the milkweed in the fall when monarchs are migrating back to Mexico. Even better, it could be replaced by other species of milkweed that are native to those areas. In short, this is a limited and manageable problem–not a complete catastrophe as some of the news articles have suggested.
Monarchs depend on milkweed to reproduce–it’s the only food source for their caterpillars. The loss of native milkweed across the US largely due to the adoption of herbicide resistant crops is one of the primary drivers of the decline of monarchs. So please, everyone, carry on. Plant your milkweed. Go to your local garden store and ask for the species of milkweed that are native to your area. If you don’t garden, support our Green Gift of milkweed which will pay for NATIVE milkweed to be planted by schools and nonprofits around the country in partnership with MonarchWatch. Planting milkweed is still the best thing we can do to help monarch butterflies.