Heading into the growing season, spongy moth (Lymantria dispar, formerly known as the “gypsy moth”) was poised to have a big year in Wisconsin. That prediction has held up and I’ve seen an influx of reports of spongy moth caterpillars and damage at the UW Insect Diagnostic Lab (IDL) this spring. In some areas, these caterpillars are causing conspicuous damage, which has also led to a flurry of questions from the public on what to do about them. Let’s take a look at how this season has shaped up, how the next few weeks could turn out, and what can be done.
What’s happened so far (as of mid-June)?
After a cool start to spring, we saw some unseasonably warm temperatures during the second week of May, which jump-started a lot of insect activity. I saw a distinct increase in diagnostic requests at the IDL around this time as well as my first reports of spongy moth caterpillars.
Initial sightings of small caterpillars mostly involved larvae dangling from trees and structures from silken threads—a dispersal mechanism down as “ballooning”. In other cases, thousands of tiny, dark caterpillars stood out against light-colored siding of homes. At first, these tiny caterpillars couldn’t cause much damage—with their small size, they simply don’t eat much. It isn’t until caterpillars are larger and more mature that they really start to chow down and damage increases dramatically. It’s estimated that 80-90% of the damage caused by these caterpillars is from the final two larval substages (instars). Reports of notable damage started to pop up a few weeks later in early June.
Based on the reports coming in to the UW Insect Diagnostic Lab, the heaviest spongy moth activity in 2022 spans from southeastern Wisconsin (Lake Geneva area) west through Rock, Green, and Dane Counties, and north to Sauk, Juneau and Monroe Counties. Overall, Dane and Walworth Counties stand out for the number of spongy moth sightings and reports of damage that I’ve received.
What will the next few weeks be like?
The end of caterpillar activity is in sight—but we’re not there yet. I’m still getting reports of spongy moth caterpillars and likely will for a few more weeks. In many cases, the caterpillars being spotted are now pretty large (1¾ – 2 inches), meaning that they’re feeding voraciously and causing lots of damage to plants. If there’s a silver lining, it’s that these large caterpillars should also be pupating in the near future—putting an end to their damage for the season. However, I’ve been receiving reports of mixed caterpillar sizes, with some caterpillars only measuring ¾ – 1 inch long. These smaller “stragglers” will continue to feed and cause damage into July, meaning we’re not entirely out of the woods yet.
Another variable that could be at play this year is a beneficial fungus known as Entomophaga maimaiga. This disease can specifically infect and kill spongy moth caterpillars and can play an important role in regulating their populations over time. Last year’s drought likely helped set the stage for 2022 by suppressing this beneficial fungus. This spring we’ve had pretty regular precipitation in many parts of the Midwest, which could help put a dent in spongy moth populations if this pathogen kicks in.
What can be done about spongy moth?
This has been one of the commonest questions I’ve been getting recently and have seen plenty of posts on social media sites like Facebook and Nextdoor asking this same question. Management of spongy moth really depends on the life stage of the insect. The UW-Madison Division of Extension Spongy Moth website has an excellent month-by-month discussion of management approaches.
For small numbers of yard trees, the burlap band method can be a way to remove larger caterpillars from the equation. However, it’s important to understand that this method can be time and labor intensive as you need to check bands daily and brush caterpillars into a container of soapy water to maximize effectiveness. [Note: don’t touch the caterpillars bare-handed, it hurts!]. For large trees, there’s not much else that an individual homeowner can do other than discussing chemical treatment options with an arborist. Many of the online posts I’ve seen have had an element of panic, but it’s also important to keep in mind that trees that are in otherwise good health can generally tolerate defoliation and will push out another batch of leaves later this year. I start to worry more about plant health when trees are defoliated repeatedly, as that can lead to secondary issues over time.
I’ve also seen a number of questions about aerial sprays for spongy moth. This year, the Wisconsin Department of Agriculture, Trade and Consumer Protection (DATCP) has been coordinating aerial spraying in the western parts of the state to slow the overall spread of this invasive species. The treatment used in early-season aerial sprays (Bacillus thuringiensis kurstaki) is most effective against small caterpillars. Later spraying will focus on disrupting the ability of adult moths to successfully find a mate. In theory, members of the public could band together to coordinate aerial spraying in their local area, but the planning process for this can take months. By the time folks were posting on social media expressing a desire for aerial treatments in their neighborhood, that option was no longer feasible.
One key thing to pay attention to later this summer will be the egg masses laid by adult female spongy moths. Each egg mass can contain upwards of 1,000 eggs, so surveying for egg masses can give insight into what the spongy moth situation could be like in 2023. Those egg masses will also remain in place for roughly nine months until they hatch next spring, which gives lots of time for a search-and-destroy scavenger hunt in your yard.
For additional information on managing spongy moths, check out the updated UW-Madison Division of Extension factsheet on this insect and the Extension spongy moth website with month-by-month recommendations.