Backyard Gardening
Removing Spongy Moth Egg Masses
By Kathryn Dorman
This past June, I wrote about the larvae, or caterpillars, of spongy moths (formerly known as gypsy moths). Those caterpillars ate and ate tree leaves, until the larvae were ready to pupate. The adult moths soon emerged, mated, and died. At present, the egg masses that the adult spongy moths produced are just sitting around, waiting for next spring. In this article, I will discuss how it might be possible to disrupt the cycle of defoliation by future spongy moth (Lymantria dispar) caterpillars.
Spongy moth egg masses can be found on any outdoor surface that is stationary long enough for the female spongy moth to deposit her eggs. Some examples include lawn, garden, and playground equipment, trees, firewood, and even buildings and automobiles. The Animal and Plant Health Inspection Service (APHIS) within the United States Department of Agriculture (USDA) provides a substantial list of places to look for egg masses at (https://www.aphis.usda.gov/publications/plant_health/fsc-moving-check-gypsy-moth.pdf).
Spongy moth egg masses are about the size of a thumbnail, are raised in the center, and are tan to light brown in color. Egg masses appear slightly fuzzy – or spongy – because the female spongy moth deposits hairs and scales from her body within the egg mass (thus the fairly new common name for this pest).
Egg masses can be destroyed from now until mid-April, before the eggs start to hatch. The University of Wisconsin-Madison Division of Extension recommends destroying the egg masses by spraying them with a horticultural oil labeled for spongy moth egg masses, which can be purchased at a garden center or online. Spray the horticultural oil onto the egg masses until they are thoroughly soaked. You could also gently scrape the eggs into a container. Be very careful not to damage the tree. Don’t just leave the egg masses on the ground. Some eggs may still survive to hatch next spring. Eggs can be destroyed by soaking them in soapy water for at least 2 days. Put the egg masses in a sealed plastic bag before placing them in the trash. You can also burn the egg masses (https://fyi.extension.wisc.edu/spongymothinwisconsin/pest-management-2/management-guide-for-homeowners/). Please note that spongy moth hairs and scales can be extremely allergenic. Be careful to wear gloves when handling the egg masses.
Many egg masses will stay hidden or out of reach. I thought there weren’t a lot of egg masses on the huge oak tree in our back yard, even though it had been completely defoliated. Then I read that the female spongy moths like to lay their eggs in protected places, like under vines. Sure enough, there were plenty of egg cases under the Virginia creeper vine (Parthenocissus quinquefolia) that was growing up the tree!
Keep trying to find and destroy those egg masses, until they hatch. At that point, other options, such as Bacillus thuringiensis var. kurstaki (Btk), may be necessary, but will not be discussed here. Why bother to even try to eliminate some of the spongy moth egg masses? Each egg case contains 600 to 1000 eggs. Every little bit can help to lessen the number of caterpillars eating the leaves off your trees next year. Even if the leaves grew back this year, a tree may not be able to survive a second year of defoliation.
Before it gets too cold outside, consider starting the process of finding and properly disposing of spongy moth egg masses. When I look at all the egg masses on our property, I know it’s going to take a lot of time and effort to get rid of them. They are all over the trees in front of our house (see Photo 1), and even on our house (see Photo 2)! There are some that I don’t think I’ll be able to reach, even if I am foolish enough to try climbing a ladder. At my age (62), I am very cautious about doing things that might require another visit to my orthopedic surgeon. I will persevere, however, because I don’t want to lose my beautiful trees!
Kathryn Dorman has been a Penn State Extension certified Master Gardener in Clinton County since 2017. She can be reached at clintonmg@psu.edu, or through the Clinton County Master Gardener Hotline at (570) 858-0198.
