While everyone is enjoying the outdoors as the weather warms up, some uninvited guests, such as spongy moth caterpillars, can spoil the fun.
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.
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Identifying spongy moth caterpillars
The invasive spongy moth caterpillars emerge from tan, fuzzy egg masses in April and feed on leaves through late June.
The caterpillars are hairy with a yellow and black head and five pairs of blue spots, followed by six pairs of red spots.
Identifying adult spongy moths
Male moths’ wings have a wavy pattern of brown to dark brown and span 1.5 inches.
Female moths are larger than males and do not fly. Wings are white and cream with wavy black markings.
Previous outbreaks
The outbreak of spongy moth caterpillars that started in 2019 and peaked in 2021 has been on the decline for 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 (DNR) 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.
The DNR provided a list of the acreage of spongy moth defoliation mapped across the state for the past five years:
- 2024: 168,879 acres
- 2023: 156,000 acres
- 2022: 386,000 acres
- 2021: 1,350,000 acres
- 2020: 947,900 acres
The Michigan DNR stated that spongy moth outbreaks are cyclical and can last up to three years. Populations will collapse due to environmental viral and fungal diseases 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.
How you can protect trees from spongy moth caterpillars
The Michigan DNR advises people to focus on areas where caterpillars, their feeding, or their droppings will cause the most significant 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. According to Michigan State University’s Enviroweather prediction model, spongy moth eggs should be finished hatching now in the Lower Peninsula.
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.
Here’s what you can do to 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.
- Caterpillar hairs can cause skin irritation, so you may want to wear gloves while managing caterpillars.
