Spongy Moth Population Declining in 2022 for Northern Lower Michigan

The Michigan Department of Natural Resources (DNR) is saying that the spongy moth population should decline this year for northern Lower Michigan.

This information comes from a NotMISpecies webinar where several experts of the species were brought together to represent nearly 100 years of knowledge. These expects include Dr. Deborah McCullough from Michigan State University, Dr. Steven Katovich of the U.S. Department of Agriculture’s Forest Service, Susie Lott of the Michigan Department of Agriculture and Rural Development and James Wieferich of the Michigan Department of Natural Resources.

The panelists said that recent aerial survey data showed much of northern Lower Michigan had experienced two or three years of defoliation, which they say typically marks the end of an outbreak cycle. Egg mass surveys in the fall of 2021 as confirms that many areas in Michigan should expect the activity of the spongy moth to decline. However, the southwest Lower Michigan have a higher change of high density populations this summer because of the findings of large, healthy egg masses.

Though they expect a low population for northern Lower Michigan, the panelist still had a few suggestions about what to do if the spongy moths return this year. This includes checking the forecast for you neighborhood by looking for healthy egg masses now before leaves expand. A healthy egg mass is larger than a quarter in size, a tan or brown color and firm to the touch. An abundance of healthy egg masses suggests a heavy infestation of spongy moth caterpillars this season.

They also gave out a few different control methods with a reminder that even if you can’t eliminate all the caterpillars, the goal is to reduce the density of caterpillars around your house. These control methods include scraping, banding and spraying.

Scraping means finding the healthy egg masses and using a scraper or hard, plastic card to scrape the egg masses into a container of soapy water overnight before putting them into a bag and disposing of them, or throwing them in a fire and burning them.

Banding means cutting a band of burlap 18 inches wide and long enough to go around the tree trunk with some overlapping. Then you tie a string around the center of the band to make a two-layered skirt around the trunk. When caterpillars climb trees daily to feed, they will get caught in the band. Then you scrape them into the soapy water.

Or homeowners looking to address an infestation in a handful of trees can buy a spray containing Bacillus thuringiensis var. kurstaki which is lethal to certain caterpillars and moths. The best time to spray is around mid-May through early June.

Other than talking about the forecast for the 2022 population and how to handle it, the panelists also took the time to explain why the population drops and how it affects other species. They explained how most of the crashes in the population is because of the Nucleopolyhedrosis virus (NPV) or the E. maimaiga fungus. The fungus lives in the soil and kills the moths when they’re in their caterpillar stage through contact with the spores. NPV naturally occurs in all spongy moth populations, but with high density populations it can spread much easier between caterpillars. These pathogens are typically the reason why the spongy moth outbreaks are usually only a couple of years, which eliminates the need for spray programs.

The experts also went on to say NPV and the fungal diseases are specific to spongy moth populations and do not affect people, pets or beneficial insects like pollinators or insect predators.

To learn more about the spongy moth caterpillars and options for Michiganders living in an outbreak area, click here.