The Delaware Forest Service has detected about 825 acres of defoliation around the Cypress Swamp, Gumboro and Nanticoke Wildlife Preserve due to gypsy moths, compared to 2021 with only about 12 acres affected by this invasive pest.

The trees most at risk of defoliation by gypsy moth, also called gypsy moth, are very old or stressed oaks. Although white oaks are the insect’s favorite tree, many deciduous tree species are often partially or fully defoliated. Healthy trees often do not die after a year of defoliation; however, severe defoliation for two or more consecutive years is of greater concern for tree health. Evergreen trees are rarely defoliated.

In the fall, defoliation can be predicted by counting the number of fresh egg masses per acre in a forest or yard. They can be found on almost any man-made or natural surface exposed to the outdoors during the months of June and July prior, so careful searching is required. Egg masses are the only life stage present in fall and winter. Fresh egg masses have a yellow-brown color, are intact and firm to the touch. Some egg masses from the previous year will persist with a whiter, bleached color, and show holes and shreds. There may also be cases of empty pupae from the previous summer that persist; however, they are not part of the insect population at this stage. More information on how to predict how much defoliation to expect next May and June can be found on the Delaware Forest Service – Forest Health page at agriculture.delaware.gov/forest-service/forest-health .

Colder, wetter weather will often lead to increased mortality from the Japanese fungus entomophaga maimaiga, the natural biological control agent for this invasive pest. Since this fungus is weather-dependent, it is difficult to predict whether it will cause a decline in population by spring 2023.

The landowner can control gypsy moth during fall and winter by removing and destroying egg masses or coating them with certain types of sprays that smother the eggs and deliver insecticides. In the spring, larval development can be stopped with various insecticides. When treating, be sure to use a federally approved product for gypsy moths and follow label directions when applying the product. The most common option for hardwood forests is to spray the young larvae aerially, usually in May when the leaves of the oaks have stretched at least an inch and the larvae are in the first and second instars of growth. Aerial spray size in contiguous hardwood forests should be 20 acres or more. Healthy caterpillars from adjacent unsprayed forests will drift to smaller areas and cause almost the same amount of damage as if they had not been sprayed. Typically, the decision to spray by air must be made by January, due to the need to contract out a spray job to the small number of licensed applicators in Delaware.

Forest owners who suspect they may have enough egg masses per acre to warrant private contract spraying can call the Forest Health Specialist at 302-698-4553 for technical assistance. Currently, the Delaware Department of Agriculture does not have an organized spray program with financial assistance.

For garden trees and small woodlands, there are many options. Control methods can be found by searching the Internet for “Control of Gypsy Moth”, but not all websites have the same level of professional review before publication. The Delaware Forest Service strongly recommends using a website created by a university extension system or a federal or state government entity.

For information on identifying gypsy moth in its egg mass, larval, pupal, and adult stages, go to the USDA Animal and Plant Health Inspection Service’s pest identification page at address tinyurl.com/23jheyuv.