Mowing the grass at certain times of the year might help boost the monarch butterfly population.
The findings of a new study, which appears in Biological Conservation, show that this kind of strategic grassland management benefits monarchs in two ways. First, monarchs lay more eggs on young milkweed—new growth after mowing. Milkweed is the sole food source for the butterflies in their larval stage. Second, fewer predators visit immature milkweed; more come during its mature stages, such as when it flowers.
“Monarch butterflies scout young milkweed to lay their eggs,” says lead author Nate Haan, postdoctoral research associate in entomology at Michigan State University. “And in terms of a food source, milkweed is more like spinach when it’s young and comparable to cardboard as it ages.”
Monarchs have declined for decades and are close to being named a threatened species. Many reasons explain their steep population decline. They face deforestation in and around their Mexican wintering grounds, increased exposure to pesticides, and lost nectar resources along their migratory routes.
In the Midwest, monarchs, in their egg and caterpillar phases, face equal challenges. In fact, any challenges have probably increased since they’ve moved out of crop fields and into grasslands where predators are more common. Katydids, ants, stink bugs, spiders, and many other predatory insects eat most eggs within the first 24 hours.
To help monarchs survive this critical window, Haan and a team of scientists teamed with the Michigan Department of Transportation, public land managers, and some private landowners to explore potential solutions.
“The habitat for monarchs is shrinking; it used to include corn and soybean fields but now it’s restricted in many places to pastures, parks, and right-of-ways along highways and interstates,” says Haan, who cowrote the paper with Doug Landis, professor of entomology.
“We found that if we mow small amounts of these areas in June or July, we see increases of anywhere from 3 to 10 times more eggs per stem on the regrowth, with fewer predators around to eat them.”
Next, researchers hope to scale up the application of strategic mowing. “We need to see how this approach affects other wildlife, such as pollinators and birds, in larger settings around the state,” Haan says.
“This could eventually lead to management recommendations to transportation departments in Michigan, and other Midwestern states, as well as landowners hoping to attract more monarch butterflies to their property.”
Anyone with milkweed in their backyard can experiment with mowing for monarchs to enhance egg laying success. Try mowing or trimming about a third of a milkweed patch in mid-June, when stems are starting to flower, and cut another third in mid-July when the mowed stems have regrown and begin to flower. Always leave the rest of the patch undisturbed.
The US Department of Agriculture, the Great Lakes Bioenergy Research Center, and Michigan State University AgBioResearch funded the study.
Source: Michigan State University