If Monarch butterflies are going to stay off the endangered species list, they need more to eat. And it’s up to us to feed them. That’s the conclusion of a new study by the U.S. Geological Survey.  One big problem: monarch caterpillars are extremely picky. They can only eat one thing — milkweed.

LISTEN: Want to Save the Monarch Butterflied? Start Planting Milkweed

Monarchs used to feast on the milkweed growing in corn and soybean fields. But then the herbicide glyphosate began to be widely used on genetically modified corn and soy fields. As a result, almost all of the milkweed is gone.

Now monarchs are relying on milkweed in other places, but there’s just not enough of it. Wayne Thogmartin, an ecologist with the USGS, estimates monarchs need 1.8 billion more stems of milkweed. So, he says, we should plant more of it. But if you do plant milkweed in your backyard, he says you should go for native varieties and avoid any milkweed that’s treated with neonicitinoid pesticides, because they can kill monarch caterpillars. 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.

This map from the USGS shows rates of glyphosate application. The herbicide kills the milkweed monarch butterflies need to survive.

The monarch butterfly population has dropped by about 80% since the mid-90s. And there’s been so much concern about the steep drop that a few years ago, the Center for Biological Diversity and other groups petitioned the U.S. Fish and Wildlife Service to protect it as a threatened species under the Endangered Species Act. The review process is supposed to take a year, but Brett Hartl, government affairs director with the Center for Biological Diversity says it often takes longer. They filed a lawsuit to get the Fish and Wildlife Service to act, and now the agency has to decide on the butterfly’s status by June 2019. But Hartl hopes that by 2019, the government, nonprofits and regular people will be able to do enough work on monarch conservation that the species won’t need to be listed as “threatened.”

Monarch caterpillars eat only one thing: milkweed. (Photo: Sid Mosdell / flickr)

Hartl says early efforts to plant milkweed and preserve habitat have been good, but monarchs need more investment — something  like the half a billion dollars the federal government has spent out west to keep the sage grouse off the endangered list.  And Hartl wonders if the federal government has the appetite to make sure monarchs have the milkweed they need to survive. The Trump administration’s proposed budget plan suggests big cuts at the U.S. Fish and Wildlife Service, the Department of the Interior and the Department of Agriculture. “That obviously has effects even on things like monarch butterflies,” he says.

Reporting by Kara Holsopple and Rebecca Williams