Monarchs’ egg-laying preferences could guide recovery efforts

Loss of habitat containing milkweed plants is considered a leading cause of the precipitous decline in monarch butterfly populations over the past 20 years. To reverse loss of this iconic species, recovery plans nationally have ambitious goals for new habitat. Iowa, in the center of the monarch’s summer breeding range, has a goal to establish hundreds of millions of new milkweed stems across 480,000 to 830,000 acres, by 2038.

Monarchs lay their tiny, off-white eggs only on milkweed, and eggs that hatch into hungry caterpillars also feed solely on milkweed leaves (though adult monarchs feed on nectar from a variety of flowers). There are hundreds of species of milkweed, over a dozen native to Iowa. But are they all equal when it comes to supporting monarch recovery?

New research by Iowa State University scientists suggests they are not, with monarch females showing a preference for certain types of milkweed when laying their eggs. The findings, from a three-year field study at multiple locations across Iowa, were published in the October issue of the peer-reviewed journal Frontiers of Ecology and Evolution, where the article, “Monarch Butterflies Show Differential Utilization of Nine Midwestern Milkweed Species,” is online at

Victoria Pocius led the study as part of her doctoral research in ecology and evolutionary biology at Iowa State. She is now continuing research on monarch behavior and physiology as a postdoctoral scientist at Pennsylvania State University.

Results from previously published laboratory research conducted by Pocius and other researchers found that monarchs prefer to lay eggs on two Iowa species, commonly known as swamp milkweed and common milkweed. Swamp milkweed is usually found in wetter habitats. Common milkweed grows in diverse conditions and spreads easily through root-like stems.

“We needed to see if monarchs’ behavior in Iowa field conditions reflected our lab findings,” said Pocius. The field study took place from 2015 to 2017, during which nine native milkweed species were established in 14 plots across Iowa on public land including Iowa State research farms and on private land.

The study results showed that, as in the lab, monarchs preferred to lay eggs on common milkweed and swamp milkweed, but they used all nine species. The researchers ranked the monarch’s use of different species and offered restoration recommendations based on monarch’s apparent preferences and other factors linked to habitats and ease of propagation.

“Choose plant species that will grow well in the conditions where you want to plant milkweed,” said Pocius. “Some species are better than others for egg-laying, but monarchs can use most, or possibly all, native species. Stay away from non-native species, like tropical milkweed, which is an annual species in Iowa and may not provide needed nourishment for monarchs that migrate through the Midwest.”

Monitoring the butterflies at so many sites over several years required assistance from trained volunteers at a number of locations. “I give our observers a lot of credit,” said Pocius. “It was gratifying to see the strong interest from those who helped at host sites, as well as from the public at our field days.”

The Iowa Monarch Conservation Consortium formed in 2015 with a goal of promoting monarch breeding and survival in Iowa. Information about the consortium, how to support monarch conservation and links to research is available at