NOTE: This story originally aired May 22, 2015.

Monarch butterfly populations have taken a big hit. By some estimates, they’ve declined by more than 90 percent over the past twenty years. Scientists say you can help monarchs by planting milkweed in your home garden. (That’s the only thing monarchs will actually eat). But if you’re not careful, you could also accidentally end up poisoning them.

The problem is that many milkweed plants you can buy at chain stores are treated with insecticides called neonicotinoids. And these are extremely toxic to caterpillars.

Don’t buy milkweeds from big box stores unless you’re certain they haven’t been treated with neonics.

Chip Taylor, director of Monarch Watch at the University of Kansas, says that when caterpillars eat leaves treated with these pesticides, the insects fall off the plant within minutes, curl up into a C shape and die slowly. These insecticides are systemic, he said, meaning they affect every part of plant.

“It doesn’t take much—two or three bites and they’re dead,” Taylor says. “We get emails very frequently, and we have to warn people: Don’t buy milkweeds from big box stores unless you’re certain they haven’t been treated with neonics. Very few of the big box stores can assure that.”

But some chain stores are slowly making changes. Lowe’s and Home Depot both say they’re working to phase out the use of those pesticides in their plants within a few years.

If consumers want to be sure they are buying milkweed that is safe for monarchs, Taylor says native plant nurseries are generally better bets. Even then, customers should ask if the plants are treated with systemic insecticides.

“Go to a native plant nursery and quiz the manager about the use of systemics. If they can assure you they don’t use any systemics, then buy the plants from them.”

Bill Schneider, who owns a native plant nursery near Lansing, Michigan, says the region has a lot of native milkweeds to choose from. There are 11 species of milkweed in Michigan; Pennsylvania has a dozen or so as well. Schneider says you can even go out and collect your own milkweed seeds from the wild.

“We go out seed collecting like some people go out garage sale shopping,” he says. “You don’t know sometimes what you’re going to find, but when you come upon it, it’s exciting.”

Schneider says it’s a good idea to do that seed collecting away from farm fields and other areas that could be sprayed with pesticides.

“We are, in a sense, trying to grow insect food,” he says. “We don’t want the plants we produce to be toxic to insects.”