LANSING, Mich. (WOOD) — West Michigan tree owners sick of caterpillar droppings raining down on them may get some reprieve this year.
James Wieferich, forest health specialist with the Michigan Department of Natural Resources, says the state agency is seeing smaller egg masses from the spongy moth, previously known as the gypsy moth. That’s a sign the virus and fungus that typically end spongy moth outbreaks are taking hold.
“We’ve done surveying in Newaygo, Lake County, Manistee, Muskegon. We’ve been over in Crawford and some of the areas of Alpena and largely the populations look to be collapsing,” Weiferich said in an April 14 webinar.
That could also mean less leaf-munching and fewer swarms.
“Largely, we’re thinking that we are seeing a collapse, that this year we’re probably not going to see a lot of defoliation in many areas. But we don’t have a crystal ball, we don’t go out to every area to look for this defoliation,” he said, adding that some spots like Presque Isle County still have a healthy spongy moth population.
Steve Katovich with the U.S. Department of Agriculture Forest Service said other indicators of a spongy moth decline are thin caterpillars hanging from trees in an upside-down “V” shape — they have the virus — and thin, desiccated caterpillars, which have the fungus.
“Hopefully you’re going to get to see more and more of that this summer in lower Michigan,” Katovich said.
UNDERSTANDING THE OUTBREAK
The latest spongy moth outbreak started in 2019 with the moth defoliating about 250,000 acres of Michigan trees, according to Wieferich. The outbreak grew in 2020 with 950,000 Michigan acres defoliated. Newaygo was among the hardest hit counties.
“And we thought that some of that might be coming to a close in ’21 and we were wrong,” Wieferich said.
He said severe heavy rains in the spring and dry conditions in the summer further stressed oak trees, which are a favorite food for spongy moth caterpillars.
“Then on top of that, we had another statewide frost at the end of May that knocked back all of our oaks and allowed this population to really thrive again in 2021,” Wieferich said.
Maps presented by the DNR show the outbreak worsened in Newaygo and Muskegon counties and stretched farther into West Michigan last year. The outbreak even reached parts of the Upper Peninsula.
“Last summer was lots and lots of places that sustained defoliation and had lots of caterpillars and lots of frass — that’s what we call the little green pellets that the gypsy moth caterpillars excrete all over,” said Deborah G. McCullough, a Michigan State University forest entomologist who has been dealing with the pest since Michigan’s biggest outbreaks in the early 1990s.
Katovich said spongy moth outbreaks and declines are cyclical. He said usually a few dry springs can trigger “big eruptions” of the moth, and outbreaks typically last two to three years until “natural enemies” curb their numbers, primarily the virus and the fungus.
“It’s not unusual for us to see this this big outbreak that you’ve been seeing in Michigan. For this insect, this is the way it behaves over time. In the intervening years, you can have very low populations, and those low populations can persist for a long time,” Katovich said.
“During the outbreaks, you can have millions and millions of caterpillars around, as many of you are probably aware of. In the intervening years, it can be hard to find a caterpillar,” Katovich said.
“Largely, if you do nothing, you probably aren’t going to deal with gypsy moths this year. I can’t guarantee that, again, because I haven’t been everywhere,” Wieferich said.
That’s also good news for trees, which can start dying from defoliation in the third year of an outbreak, especially if they’re stressed or unhealthy. Katovich said leaves from “a number of very common trees that you may have in your yard” — like oak and aspen — are among the spongy moth’s favorite to feed on. But the spongy moth will shift to white pine needles as the caterpillars grow and “become less picky.”
FROM HOPEFUL SILK SAVIOR TO TREE INVADER
Of the 450 species of invasive forest insects in the U.S., the spongy moth is the only one that was intentionally brought into the country, according to McCullough.
Unlike many invasive pests, entomologists know exactly how the spongy moth reached the U.S. According to McCullough, amateur naturalist E. L. Trouvelot brought spongy moth egg masses from his native France to his home in Medford, Massachusetts, to crossbreed with ailing silk moth caterpillars, which entomologists now know were dying from a disease caused by microsporidia.
“He thought he could make a bigger, stronger healthier silk moth and basically save the silk industry. Now biologically, that’s not ever going to work. Silk moths and gypsy moths are not related to each other at all, but I guess you’ve got to give him credit because his intentions were good,” McCullough said.
Trouvelot started raising the caterpillars at his home but it all went awry in 1869.
“The story goes that some of the caterpillars escaped out his bedroom window, became established in trees right around his house, and that’s how we ended up with gypsy moth,” McCullough said.
Trouvelot wrote a letter to the U.S. Department of Agriculture, but there’s no evidence the agency or its entomologist responded for about a decade. Then in 1889, Medford experienced the first major spongy moth outbreak which covered the Boston area.
“And people were really upset. Nobody likes those caterpillars. Nobody likes all that frass and defoliation,” McCullough said.
One of the first methods of getting rid of the moth was hiring young men to scale massive trees and scrape off as many egg masses as they could find. Other infested trees were cut down and burned. People also used a mix of lead arsenic and spray it onto trees and shrubs to try to kill the insect.
“And if you think about it, there’s really nothing good about lead arsenic. It didn’t take people long to figure out that if you cover the leaves of trees and plants with powdered lead, not to mention the arsenic, the trees and shrubs are going to die because the leaves are not getting any sun. And so what people would do is they would follow behind these big spray wagons with water and they would wash off the slurry. And so now you have lead arsenic running down the streets on the sides of the street,” McCullough said.
She said after the moth escaped and the first major outbreak, Trouvelot abandoned being a naturalist and shifted to astronomy “and he actually became a relatively important astronomer. I guess he was safer looking at the stars than trying to do biology.”
The federal government issued its first spongy moth quarantine for the Boston area in 1912. Lead arsenate was eventually replaced with DDT, then Sevin and finally bacillus thuringiensis bacteria as the favored eradication method for spongy moth.
“In terms of what you can do to reduce the density of caterpillars feeding around your home, BT is absolutely the best,” McCullough said, adding that there’s no evidence it harms native insects and it’s nontoxic to birds, mammals and humans.
HOW TO STOP SPONGY MOTHS
BT must be sprayed on tree leaves at just the right time, typically in mid- to late May when caterpillars are very young because it’s only about 50% effective after five to seven days and is less harmful to older caterpillars.
Property owners can also wrap sticky bands or burlap bags around their trees to collect caterpillars, which should be scraped off into a bucket of soapy water to die. However, you’ll need to revisit each collection site to scrape and dump more caterpillars.
Residents can also help stressed trees targeted by the caterpillar by watering them during hot, dry spells.
In late fall, the DNR advises people to scrape off any egg masses on trees and either dump them in a bucket of soapy water or burn them.
And if you still experience a swarm of spongy moths, be patient.
“They are a pain. Gypsy moth really is a people’s pest, but they do dissipate within a week or two after they start going,” Weiferich said.