Spongy Moth Forecast

For those of you who have not yet experienced the impacts of spongy moth activity…beware! For those of you who have, it should come as no surprise that the populations are forecasted to continue to rise and spread throughout Wisconsin.

According to the WI-DNR, the summer of 2023 saw a record amount of defoliation. In this area, the population was the highest in Walworth and Waukesha counties last year. The drought conditions we have experienced for the past three summers is both favorable to the populations of spongy moth and unfavorable to Entomophaga maimaiga and Bacillus thuringiensis, which are fungi and bacteria in our soil that help control the caterpillar populations. Wet spring weather may be able to boost the populations of those naturally-occurring soil fungi and bacteria.

Spongy moth (which until 2022 was termed gypsy moth) is capable of stripping a tree bare in just a few days during its destructive caterpillar stage. The caterpillars emerge from their eggs in May, but aren’t typically noticed until the middle of June. At that time, the caterpillars are approximately 1-2 inches long and their feeding is more noticeable. They can be a real nuisance! Between
the small leaf litter and the frass raining from the trees, this insect can make it hard to enjoy the start of summer.

Spongy moth is reported to feed on 500 species of plants. In our area, we see them feed the heaviest on oaks, but they’ll also chew the leaves on maples, crabapples, beeches, and cherries. Stronger, healthier trees can withstand a year or two of defoliation with limited dieback or adverse effects. However, larger mature trees, especially oaks, are not responding well to defoliation. Treatment
of these larger trees is recommended.

The best way to set your trees up for success this spring is to look for egg masses now! These orangey/tan globs are typically found in shady places: the undersides of branches, on the side of the house that gets less sun, or under awnings. You can help your trees by scraping off reachable egg masses into a bag or bin. Soak the egg masses in soapy water for 2-3 days before disposing of them. For those you can’t reach, or if the population is overwhelming, contact your Wachtel Tree Science Certified Arborist. Treatment is going to vary based on the amount of egg masses seen, location of the trees, and tree species.

Don’t wait until your tree is half eaten to seek treatment! Being proactive on treatment is the best, and often the only way to protect your trees.

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