Don't Stop Planting Milkweed

New research shows a certain type of milkweed can harm monarchs—but we still need to give these butterflies something to snack on.

January 29, 2015

Photo: Brett Whaley

If you read the headline “Gardeners’ Good Intentions Are Killing Monarch Butterflies,” you might get the impression that planting milkweed is bad for these butterflies. Well, don’t believe the hype. Monarchs need milkweed—in fact, it’s the only thing their larvae eat!

In the interest of combating that disinformation, dubbed #MonarchGate, which kicked off in the media two weeks ago, I’m going to make a few more things clear. In a hurry? Here’s the CliffsNotes version:

1. Migrating monarch populations have rebounded some from last year but are still falling off sharply.

2. The absolute best way to help them is to keep planting milkweed.

3. Select only species native to your area and seeds not treated with pesticides. Here’s a handy milkweed seed finder! (Or check out NRDC's Green Gifts.)

4. Sometimes headlines can be misleading.

Now, the full story.

A paper recently published in the Proceedings of the Royal Society B had some interesting findings about tropical milkweed. When this milkweed grows in parts of the American South, where it is not native, it lengthens the amount of time monarchs have to breed. The butterflies might then hang around too long, making them more vulnerable to a protozoan parasite called Ophryocystis elektroscirrha. The study suggests that those butterflies could potentially pass the lethal parasite to other butterflies on the way to their winter migration, when millions of monarchs congregate in Mexico.

The tropical milkweed may even encourage monarchs to skip the migration altogether. After all, why fly to Mexico when you can find milkweed in Texan gardens all year long? Little do the monarchs realize that those plants may be laden with parasites.

All this is bad news, because these butterflies are in serious decline. When the yearly population estimate came out of Mexico this week, it represented the second-lowest numbers on record.

So how much impact does tropical milkweed have on the population at large?

“While some monarchs are nonmigratory, most are still migrating to Mexico, albeit not enough,” says lead author Dara Satterfield, a PhD student at the University of Georgia. “This problem of year-round breeding and monarch disease does not explain the dramatic decline in monarchs. The decline is due to other factors.”

One big factor is a loss of milkweed plants in general, thanks to development and pesticide use. But if you go by some of #MonarchGate’s headlines—“Plan to Save Monarch Butterflies Backfires” or “How Some Well-Intentioned Americans Trying to Save Monarch Butterflies May Actually be Hurting Them”—you’d think tropical milkweed is a goddamned monarch cataclysm. It’s not.

To be fair, these headlines are technically true, but they’re way out of context.

“I think the media is taking an angle that sells newspapers and does not necessarily reflect what this important research contributes to our understanding of monarch butterflies and their conservation,” says Tyler Flockhart, a conservation biologist at the University of Guelph.

Flockhart is the scientist I spoke to last year for a story encouraging people to make milkweed seed bombs. He’s also quick to note that Satterfield and the other authors of the tropical milkweed study have been trying to do damage control of their own. Last week, they posted this Q&A to help set things straight.

The parasite problem, the researchers remind us, is limited to parts of the Gulf Coast and California. Best of all, Satterfield has an easy solution.

“We are now encouraging gardeners to plant one of the dozens of other species of milkweed native to the United States,” she says. “They naturally die back and don't allow monarchs to skip the migration.”

So the takeaway from #MonarchGate isn’t that conservationists are dooming the butterflies with good intentions. It’s that we need native milkweed now more than ever. 

“We need yards filled with it and agricultural lands making space for it,” Satterfield says.

Just don’t plant the tropical kind! Easy-peasy.

onEarth provides reporting and analysis about environmental science, policy, and culture. All opinions expressed are those of the authors and do not necessarily reflect the policies or positions of NRDC. Learn more or follow us on Facebook and Twitter.

Southeast Dispatch

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