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One of the most devastating forest pests in the United States, gypsy moth has had a presence in the country since 1869, when it was introduced accidentally by a French scientist living in Massachusetts.

Pest of the Month: Gypsy Moth

By Sean Facey


One of the most devastating forest pests in the United States, gypsy moth has had a presence in the country since 1869, when it was introduced accidentally by a French scientist living in Massachusetts.  By 1902, it had spread throughout New England, into eastern New York and northern New Jersey. Currently, known gypsy moth infestations stretch from east in Massachusetts to west in Wisconsin and from north in Maine to south in Virginia.

The larval stage (caterpillar) of gypsy moth causes the most damage to trees. Gypsy moth is a defoliator, and repeated years of attack cause significant damage to trees, leading to additional infestations, diseases and death.


Host material

According to Penn State’s Entomology fact sheet on Gypsy Moth, host trees include: “alder, [ital>Alnus<ital] spp., aspen, Populus spp., gray birch, Betula populifolia, white birch, B. papyrifera, hawthorn, Crateagus spp., larch, Larix spp., linden, Tilia spp., mountain ash, Sorbus spp., oaks, Quercus spp., Lombardy poplar, Populus nigra, willows, Salix spp., and witch-hazel, Hamamelis spp. Plants favored by older larvae but not by young larvae include beech, Fagus spp., red cedar, Juniperus spp., chestnut, Castanea spp., hemlock, Tsuga spp., plum, Prunus spp., pine, Pinus spp., and Colorado blue spruce, Picea pungens.”



For leaf-chewing caterpillars, the obvious symptom is skeletonized or mostly consumed leaves. The caterpillar itself can often be visibly observed feeding on the leaf tissue.

Gypsy moth is a distinctive-looking caterpillar. It has a mostly black body with five sets of blue dot pairs and six sets of red dot pairs along its back. It also has urticating hairs, giving it a “furry” appearance. These hairs may cause skin irritation on humans, especially on children.


Damage caused by gypsy moth

The main type of damage from gypsy moth is caused by the caterpillar stage of larvae chewing the leaves of host species. Defoliation levels up to 30 percent are considered mild, and the tree is generally able to recover from attack. Damage over 51 percent is considered serious, and the tree has a lower chance of survival. Gypsy moth outbreaks tend to be cyclical, with two years of low defoliation followed by two years of severe defoliation. After the fourth year of gypsy moth presence in the area, the population is greatly diminished but may return in subsequent years.


Management options

Monitor trees for injury by gypsy moth, often seen as pin holes or notching in the leaf. Apply formulations with the active ingredient acephate (such as ACE-jet) as soon as symptoms of injury or as the caterpillars themselves become visible. Depending on species, infestations occur in spring or early summer. Timing is important as the formulation will only remain active in the tree for three to five weeks.

For longer-lasting control for Gypsy Moth, TREE-äge insecticide (emamectin benzoate) may be applied by an applicator with a state-approved pesticide applicator’s license. TREE-äge is labeled for up to two years of control for gypsy moth. TREE-äge may be applied as long as air temperatures are below 90 degrees Fahrenheit and above 40 degrees Fahrenheit and the trees are actively transpiring. The benefit of TREE-äge over ACE-jet is flexibility in timing and longer residual control.

Homeowners can easily assist in managing gypsy moth by destroying the egg masses and larvae when present. Egg masses are a light tan color, average 1-1/2 inches long and 3/4 inch wide, and contain up to 1,000 eggs per mass. According to the USDA-APHIS, “The removal of the egg masses from their locations can be performed easily with a putty knife, stiff brush, or similar hand tool. Simply dispose of the egg masses in a container of hot, soapy water, or place them in a plastic bag, seal it, and set it in the sun.” Crushing the larvae will suffice.


Sean Facey is the strategic accounts and support services manager at Arborjet with 30 years of experience in various facets of the green industry. Applying his skills in arboriculture, entrepreneurship, public policy, and international sales management, Sean dedicates himself to Arborjet and the preservation of the natural and urban forests.


* Important: Always read and follow label instructions before buying or using these products. The instructions contain important conditions of sale, including limitations of warranty and remedy. TREE-äge Insecticide is a Restricted Use Pesticide and must only be sold to and applied by a state certified applicator. TREE-äge is a registered trademark of Arborjet, Inc. TREE-äge is not registered for use in all states. Please check with your state or local extension service prior to buying or using this product.




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