First the sidewalk on Oakland Drive near Western Michigan University was covered with nature’s gunk, dead caterpillars and something called “frass,” the word for insect feces. Now several grand oaks on Oakland have lost their leaves.
It’s the work of an invasive insect, the spongy moth caterpillar. And it’s a nuisance for homeowners and tree lovers alike.
The immature “larval” stage of the spongy moth voraciously feeds on the leaves of more than 300 species of trees from May to early July. They especially like oaks, and can leave host trees heavily or even completely defoliated by July.
The Michigan State University Extension has some good news, the current season will be over by mid-July. Soon the caterpillar’s will have grown into mature moths, and mature moths don’t eat anything. They just mate, lay eggs on tree bark, and die.
And there’s more good news. Based on aerial survey data from Michigan’s Department of Natural Resources, much of Michigan’s Lower Peninsula has experienced two or three years of defoliation. That’s a good sign because outbreak cycles typically last just two to three years.
“Oaks, most hardwood trees can easily take a season of spring defoliation two years in a row with no problem,” said Jeremy Jubenville, a greenhouse agent and entomologist who works at the MSU Extension in Kalamazoo.
Jubenville said the spongy moth was imported from Europe by an entomologist in Massachusetts in 1869. He planned to breed it with a silk moth for a more robust source of silk. But then some adults escaped and laid eggs in the entomologist’s yard, and Jubenville said, “the rest is history.”
By the mid-1980s the moths had migrated to Michigan, first in Clair and Midland counties. Since then, spongy moths have established themselves in all of Michigan counties, leaving homeowners wondering what to do.
“Don't panic. They're gonna go away,” Jubenville said. “I understand. There are some owners, especially homeowners if they have lots of hardwoods and they can just hear the frass raining down and you know, their deck is covered with poop. I know that can be sometimes very difficult to live with. But I would urge just a little bit of patience, and especially at this point in the year, toward July 4, through the first couple of weeks of July it's all going to be over.”
If you suspect your trees are infected with spongy moths, beyond patience, Jubenville recommended watering instead of spraying chemicals. That’s because hardwood trees may set new buds and regrow the entire canopy of leaves before summer ends.
“If homeowners want to do something to help them, they can water them because defoliation is a stress, especially in times of drought and we're in a pretty dry, dry period,” Jubenville said.
“That will help them because trees still need water, cells need, you know, plant cells need water for cell expansion, that sort of thing.”
Another thing Jubenville recommended is to remove the spongy looking egg sacks on the trunks of trees in the fall.
“We recommend doing it in the fall, like in September or October,” Jubenville said. “It’s because there are other parasites out there. There are wasps out there that will, like to, they’ll feed on the caterpillar eggs themselves. Right, and we want to let nature just take, or do most of the work for us because it’s free and it’s easy.”
Another reason to wait until the fall is to make sure you are removing new egg sacks. Spongy moths lay their eggs in late summer.
If you find spongy moth caterpillars on evergreens, Jubenville recommended calling the extension service for more help. Spongy moths can cause more damage to conifers than other trees. The number to the Kalamazoo office at 269-383-8830.
More information on spongy moths can be found on the MSU Extension website.
