If it looks like the picture below, it’s a spongy moth (Lymantria dispar), formerly known as gypsy moth. Spongy moth is an invasive insect that is not native to North America. Even so, they have been established in Michigan for more than 40 years. There are a few characteristics to look for to identify spongy moth. They have blue and red spots going down the back, hairs along the side, and a yellow head with dark markings. 

Image credit: Karla Salp, Washington State Department of Agriculture, Bugwood.org

Spongy moths are voracious feeders. They prefer to eat oak leaves, and in high numbers can completely eat the leaves off a forest of oaks. Other highly preferred host trees include aspen, willow, basswood, apple and crabapple. When most of the leaves on oaks or other preferred hosts are gone, the caterpillars may move to less suitable trees like maples. When they’ve exhausted all other options they may even feed on needles of spruce or white pine trees.

The good news is that oaks and other hardwood trees are able to survive this even complete defoliation. They will set a second set of buds and new leaves will begin growing, usually around mid-July. By then, the caterpillars will have finished their feeding. Oak trees do die sometimes, but that typically occurs only when the trees are already declining and stressed by other factors. Hardwood trees can tolerate at least 3 to 4 years of heavy feeding. After that, the density of the moth population will drop. Unfortunately, spruce, pines and other conifers are not able to grow back their needles, and can be killed if they lose almost all their needles.

No! Spongy moth populations are controlled by two diseases; one is a virus and the other is a fungus. These two diseases won’t completely eliminate spongy moths, but their densities will drop back to very low levels. The virus is particularly important for controlling outbreaks. It builds up in populations and by the third or fourth year of an outbreak, most caterpillars are likely to be infected and die. The fungus requires specific weather conditions in spring. Unfortunately, it takes a few years for the pathogens to build up enough to kill the moths. Most of the outbreaks in northern lower Michigan began collapsing in 2022 or 2023. In southwest Michigan, populations of spongy moths should crash in 2023 or at least in 2024.

Spongy moths are rarely a concern for woodlots and forests. Although a well-stocked woodlot can look bare for a few weeks after a heavy spongy moth infestation, most trees will recover. Trees that are already severely stressed by drought, disease, flooding, etc. then it is less likely to recover. 

As noted above, spongy moth populations are naturally managed by two diseases, so in most cases, control measures aren’t necessary. If the moths are falling into the recreational or ornamental areas around your home, you may try to cut back their densities. In fall or winter you can scrape the spongy, tan colored egg masses into a bucket of soapy water and let them soak for a few days. You’ll probably only want to do this on a few trees near your house, as it can be a time consuming process. You can also wrap a band of burlap around the trunk of trees near your house in spring, when the caterpillars are small. Caterpillars do most of their feeding at night and will hide under the band during the day. You can knock the caterpillars off the tree and into a bucket of soapy water every day. While this is not practical for too many trees, it can help reduce spongy moth numbers on trees around your home.

A few insecticide options also exist, which are most effective in early spring when the caterpillars are still very small. More information about control options can be found at https://www.canr.msu.edu/ipm/Invasive_species/gypsy-moth/index

Visit the Michigan State University spongy moth website. Lots of information about the biology of spongy moths and options for managing them on your property is available at https://www.canr.msu.edu/ipm/Invasive_species/gypsy-moth/index