You should see fewer spongy moth caterpillars this year: What to know

Experts think we’re at the end of a spongy moth outbreak cycle

Egg mass decline: Egg masses smaller than a quarter are an indication that the NPV virus is causing a decline in spongy moth populations. (Bill McNee, Wisconsin DNR,

You might have seen them in your backyard last year, the spongy moth (formerly gypsy moth), but will they be back again this year?

Spongy moth populations were high last year across lower Michigan, according to the DNR. Experts got together on April 14 to answer questions during a NotMISpecies webinar.

What experts expect for 2022

Experts said recent aerial survey data shows much of northern lower Michigan has experienced two or three years of defoliation, which usually marks the end of an outbreak cycle.

Egg mass surveys in the fall of last year confirmed that many areas should see less spongy moth activity. That means fewer caterpillars this year. Although, they did note that areas like Jackson County and parts of southwest lower Michigan had large, healthy egg masses and could see high density populations again.

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Last large-scale outbreak occurred in the 1990s

The last large-scale outbreak occurred in Michigan from 1992 until 1996. Since then there have only been occasional outbreaks in certain areas. There have been continuous efforts to keep the spongy moth population in check.

NPV: Dead caterpillars in an inverted “V” position are a sign that the NPV virus is at work. (David Cappaert,

Keeping spongy moth populations down

The fungal pathogen, Entomophaga maimaiga, was deployed in 1991. Experts said it was killing spongy moth caterpillars in the northeastern states.

The fungus remained in the soil from year to year and infected spongy moth caterpillars that came in contact with it. Caterpillars affected by the fungus remain attached to tree trunks and hang straight down.

Nucleopolyhedrosis virus also helps keep populations down, it occurs naturally in all spongy moth populations. It spreads through contact between caterpillars during outbreaks. You can tell if a caterpillar has been infected because dead caterpillars infected with NPV are attached to tree trunks in an upside-down V position.

Both the NPV virus and E. maimaiga fungus can affect the same population, and dead NPV and fungus-killed caterpillars can be on the same tree.

Officials say the pathogens eliminated the need for spray programs. Spongy moth outbreaks typically peak every seven to 10 years. In those years, the virus and fungal disease spread more easily and eventually cause the population to crash.

Do NPV and the fungal pathogen affect other species?

No. NPV and the fungal disease do not affect people, pets or other insects. They are specific to the spongy moth. They also remain in the environment and keep the spongy moth population down over time.

NPV has been able to infect a few other species but only in a lab setting. Native species either can’t be infected or have other behavioral patterns that prevent them from being infected.

Why isn’t there a spray program?

Michigan does not have a statewide spray program as the spongy moth is a “naturalized pest.” Some areas have millages in place to survey and spray residential areas when necessary.

While an outbreak is unpleasant, it is rarely a problem for healthy trees and forests.

Old egg masses: Egg masses that are light in color, falling apart or filled with tiny holes are old and will not hatch caterpillars this year. (Milan Zubrick, Forest Research Institute Slovakia,

What can I do if spongy moth returns this year?

The first thing the DNR wants you to do is look for healthy egg masses now, before leaves expand.

Healthy egg masses are larger than a quarter and tan or brownish in color. They are also firm to the touch. If there are few egg masses or they are smaller than a quarter, that indicates the population is collapsing because the NPV pathogen is increasing.

Old, no longer productive egg massives are often found after an outbreak year and should not be counted. Those masses are usually whiteish in color and may be falling apart or have pin-size holes in the mass.,

First instar: Hundreds of newly hatched spongy moth caterpillars swarm around an egg mass. (Bill McNee, Wisconsin DNR,

How to reduce the density of caterpillars around your house

  • Scraping: If healthy egg masses are found on trees, buildings or outdoor furniture around the home, act now, as egg masses can start hatching anytime! Use a scraper or hard, plastic card to scrape egg masses into a container of soapy water. Let them soak overnight, then bag and dispose of them. Alternately, egg masses can be placed in a fire and burned.
  • Banding: Cut a band of burlap 18 inches wide and long enough to go around the tree trunk and overlap a bit. 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. Scrape them into a bucket of soapy water to kill them.
  • Spraying: To address an infestation in a handful of individual trees, homeowners can purchase a spray containing Bacillus thuringiensis var. kurstaki, a bacterium that naturally occurs in the soil but can be lethal to certain caterpillars and moths. The best time to spray is when caterpillars are small, usually mid-May through early June. Be sure to choose an Environmental Protection Agency-registered pesticide and apply it according to label directions. Remember, there is no good reason to spray woodlots or forested areas. Healthy trees and forests can withstand periodic infestations.

To learn more about spongy moth caterpillars and options for residents living in an outbreak area, visit the MSU IPM Gypsy Moth website. The bulletin Btk: One management option for Lymantria dispar offers detailed information about Btk management for spongy moth.

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About the Author:

Kayla is a Web Producer for ClickOnDetroit. Before she joined the team in 2018 she worked at WILX in Lansing as a digital producer.