Spongy Moths and Their Impacts in Michigan

Spongy moths, formerly known as gypsy moths, are an invasive insect that can cause serious damage to Michigan trees and forests. The greatest harm occurs during the caterpillar stage, when larvae feed heavily on leaves in spring and early summer. Repeated infestations can weaken trees and leave them vulnerable to additional issues.

Spongy moth caterpillars feed on the leaves of many common tree species, including oak, maple, birch, willow, and aspen. During an outbreak, they can strip a tree of most or all of its leaves—a process called defoliation. While healthy trees typically survive one year of defoliation, repeated stress over several seasons can significantly impact long-term health.

When a tree loses its leaves, it also loses its ability to produce food through photosynthesis. To recover, it must use stored energy to produce a second set of leaves later in the season. This energy drain reduces growth and weakens the tree’s natural defenses. As a result, stressed trees become more susceptible to disease, root issues, and secondary insect pests.

Damage from spongy moths goes beyond leaf loss. Weakened trees are more likely to develop dead branches, poor canopy structure, and reduced vigor. In severe cases—especially when combined with drought or other environmental stress—repeated infestations can lead to full tree decline or death.

Homeowners may notice signs of spongy moth activity such as chewed leaves, caterpillars hanging from silk threads, and dark droppings (called frass) collecting on patios, decks, or beneath trees. Egg masses, which appear as tan, fuzzy patches on tree trunks or outdoor surfaces, are another clear indicator. Recognizing these early signs can help prevent further damage and guide timely tree care decisions.

A few simple homeowner actions can also help reduce spongy moth impacts. Scraping and disposing of egg masses during fall and winter can lower next year’s caterpillar population. Placing burlap bands around tree trunks in late spring can trap caterpillars and make them easier to remove. Keeping trees well-watered and mulched helps them recover from defoliation and maintain vigor. While these steps can’t eliminate an outbreak entirely, they can meaningfully reduce stress on your landscape trees.

Managing spongy moth damage often involves monitoring populations, removing egg masses, and, when necessary, working with a certified arborist. Early detection and proper care can help protect trees and reduce long-term impacts. During drier, hotter months, deep watering—at least once a month for mature landscape trees—can help combat drought stress on top of caterpillar damage. Consult with a local arborist to determine which affected trees only need monitoring and which may need to be planned for removal. Trees in decline should be approached with caution and removed only by experienced professionals using appropriate equipment such as a bucket truck or aerial lift.

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