Though their population has been on the decline, spongy moth caterpillars continue to be a nuisance.
Spongy moth caterpillars cluster on oaks and other deciduous trees and eat their leaves, leaving the trees vulnerable to disease and other pests. During large outbreaks of these caterpillars, debris and the BB-sized pellets of frass (droppings) they leave behind can be a nuisance.
The outbreak of spongy moth caterpillars that started in 2019 and peaked in 2021 has been on the decline the last few years. In 2021, spongy moth caterpillars did 1.3 million acres of damage. In 2022, they did 386,000 acres of damage.
In 2023, the Michigan Department of Natural Resources conducted aerial surveys of nearly 20 million acres across Michigan and mapped 156,000 acres of defoliation, or leaf loss, due to spongy moth caterpillar feeding.
Spongy moth caterpillar expectations for 2024
In 2024, defoliation is expected in areas of Allegan, Barry, Kalamazoo, Muskegon, Ottawa and Van Buren counties. Those areas have seen heavy infestations over the last three years. Isolated areas of the central and western Upper Peninsula could also see defoliation.
“Since the major outbreak in the early 1990’s, spongy moth populations have become cyclical, thanks in part to viral and fungal agents in the environment that spread quickly when caterpillar numbers are high, ultimately causing a population crash,” said Cheryl Nelson, DNR Forest Health forester. “While spongy moths don’t disappear entirely, the nuisance level is reduced to unnoticeable levels, allowing deciduous trees to enjoy eight to 12 years of healthy growth before the population builds again.”
According to Michigan State University’s Enviroweather prediction model, spongy moth eggs should have started hatching in parts of the Lower Peninsula last moth.
Spongy moth caterpillars feed for about six to eight weeks and usually stop in midsummer.
How can you protect trees from spongy moth caterpillars
The Michigan DNR says people should focus on areas where caterpillars, their feeding, or their droppings will cause the biggest problems.
Homeowners should check their decks, outdoor furniture, fences, and trees for egg masses. Egg masses can be scraped into a bucket of soapy water, burned, or buried.
Young caterpillars can be swept into a bucket of soapy water with a whisk broom. They should be soaked overnight before their remains are disposed of.
Another step you can take is to make a tree trunk trap. You can do this by cutting a band of burlap 18 inches wide and long enough to go around the tree trunk and overlap a bit. You then tie a string around the center of the band to make a two-layered skirt around the trunk. When caterpillars climb trees to feed they will get caught in the band. Scrape them into a bucket of soapy water to kill them.
Pesticide is another option. The Michigan DNR suggests using Btk (Bacillus thuringiensis var. kurstaki) which is very effective on spongy moth and not as damaging to other insects. Btk works when caterpillars eat it, so it should be put on tree leaves, not directly on the caterpillars. It is most effective on young caterpillars and should be used within one to two weeks of hatch.
The DNR said you should focus your efforts on protecting young or vulnerable trees, or trees most important to your landscape. Watering these trees at least once a week can also help keep them alive. Large trees should be treated by a certified professional.
Hardwoods will usually releaf in mid- to late July, even after suffering more than 50% defoliation.