Spongy moths are waning, but keep an eye on your trees (and your picnic table)

The weather is warming up in Michigan, which means it is time for beach days, picnics, barbecues and fun times outside. While everyone is welcome to enjoy the outdoors, some uninvited guests such as invasive spongy moth caterpillars can make this time of year less fun.

These hungry caterpillars group together on deciduous trees such as oaks. They can strip them of leaves, and they release BB-sized pellets of frass (droppings) on whatever is below.

The most recent outbreak reached its peak in 2021 and has been declining for the last few years. In 2025, we can expect to see some defoliation in southwest Michigan, but that population should be declining. There may be some small pockets of scattered defoliation in the Upper Peninsula, but the U.P. may see more defoliation from forest tent caterpillars than spongy moths. Native forest tent caterpillars emerge before spongy moths, but they can be found feeding on some of the same species.

The Department of Natural Resources has been keeping watch over the defoliation, or leaf loss, by surveying 20 to 24 million acres annually from the air. Acres of spongy moth defoliation across the state for the past five years are below:

• 2024: 168,879 acres
• 2023: 156,000 acres
• 2022: 386,000 acres
• 2021: 1,350,000 acres
• 2020: 947,900 acres

According to Michigan State University’s Enviroweather prediction model, spongy moth eggs should be hatching now in the Lower Peninsula. If spongy moth populations are a concern in your area, there are multiple steps you can take to reduce their impact.

Treatment options

In areas where populations are high, it is important to remember that healthy trees will rebound, so management efforts should be concentrated in areas where the caterpillars will cause the most problems.

• Before they hatch, inspect decks, outdoor furniture, fences and trees, focusing on your favorite outdoor areas, for tan colored, fuzzy egg masses about the size of a quarter. Scrape any accessible egg masses into a bucket of soapy water or burn or bury them.
• After hatching, use a broom to sweep young caterpillars into a bucket of soapy water. Soak them in the water overnight and then dispose.
• Create a tree trunk trap: Cut a strip of burlap 18 inches wide and long enough to go around the tree trunk and overlap. Tie a string around the center of the band to make a two layered skirt around the trunk. When the caterpillars climb the tree to eat, they will get caught in the band. You can then scrape them into a bucket of soapy water and dispose.

If you decide to use a pesticide, use one containing Btk (Bacillus thuringiensis var. kurstaki).  This is a highly effective pesticide for spongy moths, but it has little impact on beneficial insects. This pesticide works when the caterpillars ingest it, so it should be applied to the leaves of the trees. It works best on young caterpillars and should be used within one to two weeks of hatching. Be sure to follow the label instructions to ensure safe pesticide application.

Don’t panic, stay safe!

If you are in an infested area, know that it is impossible to completely get rid of all spongy moth caterpillars. Consider cost, safety and environmental health when you are deciding how to handle the insects.

• Stay safe! Avoid climbing trees or using ladders on any unstable surfaces to get to egg masses.
• Focus on protecting young and vulnerable trees first. Watering at least once a week can help trees keep their vigor.
• Do not overuse pesticides. Choose the right product in the right amount for the situation by following the label instructions. In Michigan, the label is the law.
•  If you feel it is needed, have larger trees treated by a certified professional.

  Remember…

• Spongy moth outbreaks are cyclical and last up to three years. Populations will collapse due to environmental viral and fungal pathogens and remain at unnoticeable levels for eight to 12 years.
• These caterpillars feed for about six to eight weeks, usually slowing in midsummer.
• Hardwoods that are affected will usually releaf in mid to late July.

More information on spongy moth is available at Michigan.gov/Invasives.

A recording of the 2022 NotMISpecies webinar, “New name, familiar pest: Preparing for Lymantria dispar (formerly known as Gypsy moth)” is available for viewing at Michigan.gov/EGLE/Outreach.

If further information is needed, email questions to DNR-FRD-Forest-Health@Michigan.gov. Note that responses may be delayed due to high volume during the spring and summer.