Gypsy Moths

By taking a few easy steps, you can help protect trees in WFB from the threat of gypsy moths, which are present in SE Wisconsin. Fall is an important time to destroy the eggs that gypsy moths have laid that will hatch in spring. Each egg mass destroyed is 600-1,000 fewer caterpillars that won’t be attacking leaves on trees next year. According to the DNR, gypsy moths are particularly fond of oak, white birch, mountain ash and linden trees. They can also be found on maples and chestnut trees.

Egg Masses


Gypsy moth egg masses are often found in cracks or hidden spots. On trees, they can be found in cracks in bark, on the underside of large branches, under peeling bark or in holes. Log or rock piles are other favorite spots to hide egg masses. Female moths will also place egg masses on lawn furniture and play sets, behind shutters, beneath lower rows of shingles and under soffits on homes and garages. Egg masses are typically teardrop shaped and about 1 to 2 inches long. They are a buff yellow-brown color, similar to the color or a manila folder. They look like they’re made of a rounded pad of felt and are firm to the touch. The mass will persist after the eggs have hatched in May and then are bleached, often torn, and soft and spongy to the touch. If you find any masses you should take action (spray or scrape) by mid-April before the larvae begin to hatch.

Physically remove the egg masses - Scrape the egg masses into a jar and microwave on high for two minutes, or cover them with soapy water for at least two days to kill the eggs. Be sure to kill the eggs, as masses just scraped onto the ground and crushed under foot can survive. Wear gloves when handling egg masses as the hairs in the “felt” can cause a skin rash.

Spray egg masses with special oil - The UW-Extension recommends Golden Pest Spray Oil from Stoller Enterprises, Inc. The active ingredient in this product is soybean oil and it acts by coating the eggs and suffocating the larval embryo within. GPSO also includes dispersants that keep the oil in a fine emulsion so that it penetrates the egg mass well. Soybean oil by itself won’t penetrate the egg mass hairs. GPSO can be sprayed directly on gypsy moth egg masses from late Fall through mid-April whenever the temperature is above 40 degrees Fahrenheit. It is available locally at garden centers. The DNR has an excellent pamphlet regarding gypsy moths that can be found online. For further assistance, residents should contact a tree care company.