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Gypsy Moth

Gypsy moth (Lymantria dispar) is a serious forest pest and is responsible for killing millions of oak and other species of trees across the state. Although oak species are preferred, gypsy moth caterpillars feed on hundreds of other tree and shrub species, including:

  • Apple
  • Alder
  • Aspens
  • Basswood
  • Birches
  • Hawthorn
  • Hemlock
  • Tamarack (larch)
  • Pines
  • Spruces
  • Willows
  • Witch hazel

It usually takes more than one year of defoliation before trees die, however, conifers that are defoliated may be killed after a single season of defoliation.

Although the boom and bust cycles of the gypsy moth are less severe than during the past, they still require control during years when their populations are high.

Management of Gypsy Moth Infestations in Pennsylvania Forests

Perhaps the longest-standing effort to manage forest pests on Pennsylvania's forest lands has been the DCNR's Bureau of Forestry's gypsy moth program.

The gypsy moth has been causing significant forest damage in Pennsylvania since the 1970s. The most recent outbreak occurred from 2013 to 2019 and another outbreak is building for 2021. This pest has been the principal agent of tree mortality on state forest land since the 1970s.

The DCNR Bureau of Forestry uses an integrated pest management approach to monitor and treat gypsy moth populations to lessen tree mortality and prevent significant defoliation.

The bureau conducts annual egg mass surveys to monitor gypsy moth populations and implements a suppression program when populations exceed threshold levels.

The bureau uses applications of Bacillus thuringiensis, subspecies kurstaki (Btk), a natural biological insecticide, and tebufenozide, an insect growth regulator, to control gypsy moth populations via aerial application using airplanes and helicopters.

Tips for Homeowners with Gypsy Moth Infestations

Survey your property for egg masses in the summer and fall. The gypsy moth has one generation per year in Pennsylvania.

Females lay their eggs as light tan egg masses (100-1,500 eggs/mass) on trees, stones, and other substrates during June and July. Eggs hatch from mid-April to early May the following spring.

Mechanical Removal of Gypsy Moth Egg Masses and Larvae

Tactics for mechanical removal of gypsy moth egg masses can be effective for individual yard trees but are not effective as a forest-wide control method. Methods include removal of egg masses before they hatch and removal of unnecessary yard objects where egg masses can be hidden, such as:

  • Piles of old wood
  • Building materials
  • Dead branches, firewood, and other refuse

Egg masses should be scraped into a sealed container or bag and disposed.

Another control tactic is wrapping burlap around the trunks of trees where gypsy moth larvae can hide during the day.

The larvae hiding under the burlap are then scraped into a can of soapy water, killing the larvae.

Insecticide Treatments to Control Gypsy Moth Caterpillars

Using insecticides to reduce defoliation during high gypsy moth densities is an effective option; however, they do not eliminate the gypsy moth entirely or shorten the infestation period.

Homeowners must assess the risks and benefits of insecticide use. Insecticides are not necessary unless the population of gypsy moth egg masses or larvae indicate a threat to your trees.

There are several insecticides registered for use for gypsy moth; however, only some of them are available for homeowner use. Always read the label instructions before using an insecticide.

Gypsy Moth Spraying for Private Residential Landowners

DCNR conducts an aerial gypsy moth suppression program to treat state and federal forest lands.  In 2021, private landowners or municipalities can conduct their own treatment program by following the steps in the document "Guide to Conducting a Private Gypsy Moth Suppression Program." 

In addition, a list of aerial applicators (PDF) licensed to work in Pennsylvania is provided.

Treatments begin in the spring (usually May) when 50 percent of the caterpillars are in their second instar, so timing is critical. 

If you believe that you have a need for a gypsy moth suppression treatment, you should begin making plans during the fall and winter months preceding the year of treatment.