Researchers Say Largest Monarch Butterfly Population In Over A Decade Could Migrate To Iowa

May 2, 2019

The largest population of monarch butterflies in over a decade could be headed to Iowa later this spring, according to Iowa State University researchers. Monarch butterflies covered almost 15 acres of forest canopy in Mexico last winter and are on the move north.

During winter 2013 to 2014, the monarch butterfly population hit a low point, covering fewer than 2.5 acres of forest canopy in Mexico. But this past winter, researchers observed the largest population since 2007. Roughly 225 million adult monarchs could come to Iowa later this spring.

That’s because the weather conditions were favorable with a mild winter in Mexico and the southern U.S., said ISU toxicology professor Steve Bradbury, who works in the Department of Natural Resources, Ecology and Management.

“It’s like the three bears. The temperature’s just right, the wind direction’s just right, the amount of rainfall is just right,” Bradbury said. “So there’s plenty of milkweed and nectaring plants.”

If the population is as big as expected, scientists will want to maintain that. The state will need more summer breeding habitat like patches of milkweed and native forbs to maintain a steady population, Bradbury said.

“If these numbers coming up are as high as we think they might be, we could be overwhelming the amount of milkweed we have in the upper Midwest and Iowa,” Bradbury said. “What we want to do is build our habitat bank, if you will, in Iowa, up to the point that we can maintain those high numbers of monarchs.”

That means adding about 480,000 to 830,000 acres of habitat over the next 10 to 20 years, he said. The Iowa Monarch Conservation Consortium, which is made up of 45 organizations across the state, plans to do that by 2038.

Bradbury says protecting existing habitat will also help maintain the population. He said that includes using pesticides responsibly and avoiding mowing patches of milkweed.

“It’s very helpful if folks in Des Moines and Sioux City in their gardens are getting habitat patches started, and county parks and city parks, renewable fuel facilities getting their patches in," Bradbury said. "It all combines and is important."

ISU researchers plan to study monarch populations this summer. Bradbury said they will be putting very tiny radio transmitters on adult monarchs so they can better understand their flight patterns and how close they need to be to milkweed or a nectar plant before they detect it.