Gypsy Moths

In a recent blog post, I discussed the benefits of a warm and dry period in the spring that was beneficial for the development of acorns. We received numerous comments since that posting about a different benefactor of that warm, dry spell this spring. It is causing a bit of concern.

gypsy moth caterpillar

Gypsy Moth Introduction

The gypsy moth was accidentally introduced into Massachusetts in 1869 by a French scientist trying to find a disease-resistant caterpillar to increase the output of silk. Since first being detected in Pennsylvania in Luzerne and Lackawanna counties in 1932, the gypsy moth has wreaked havoc on Pennsylvania’s forests, killing millions of oak trees along the way. The gypsy moth is now one of the most destructive insects in the eastern United States. It is one of many foliage-eating pests that cause an estimated $868 million in annual damages in the United States.

Effect on State Game Lands

State game lands suffered tremendous impacts from gypsy moths in the 1980s and 1990s. Tens of thousands of acres of oak forest experienced significant defoliation. Many of these areas had salvage harvests done to make use of the dead timber, which combined with other factors, helped to regenerate forests that were no longer dominated by our important oak species. The management of these declining oak forests is still one of the most pressing challenges facing foresters today.


Defoliation that occurs in multiple years can often stress trees to the point where mortality can be noticed on a large scale. Therefore, pest suppression efforts for gypsy moth are an important part of active habitat management on our state game lands. In a recent outbreak in 2008-2009, state game lands suffered more than 100,000 acres of severe defoliation. Funding constraints only permitted us to spray the most important areas. In 2008, pest-suppression treatments occurred on more than 43,000 acres of state game land forests at a cost of more than $1.3 million. In 2009, the Game Commission spent another $554,000 to spray more than 22,000 acres of state game land forests. Even though we have not seen a widespread outbreak since that time, our habitat managers have been vigilant in conducting yearly suppression efforts ranging from 5,000 to 13,000 acres of state game land forests across the entire state.

gypsy moth egg masses.Bugwoodorg
Gypsy Moth Egg Masses Photo:


Gypsy moth outbreaks seem to run on a 10 to 12 year cycle. We cannot afford to spray every affected acre, so we must try to anticipate where the gypsy moths will show up (based on what we have seen in the past). We then try to direct active forest management so that healthy regeneration of desired species can be established before widespread mortality makes it much harder to do so. The Game Commission also collaborates with the Department of Conservation and Natural Resources and the United States Forest Service on research projects to try to identify other ways to combat this ever present and destructive pest of Pennsylvania’s forests.

Natural Control

Spraying insecticides is not the only help we have in controlling gypsy moth populations; natural factors are also very important. Since the 1980s, the fungus Entomophaga maimaiga has had a large impact on gypsy moth populations. This fungus needs cool and wet weather to negatively impact gypsy moth populations. The caterpillar larvae can feed and may cause some defoliation, but they usually die before completing the life cycle resulting in much fewer adult insects and egg masses. However in years with warm and dry periods that coincide with leaf-out in the spring, the caterpillars can do massive and widespread damage because the fungus is not as prevalent in the forest. NPV, or nucleopolyhedrosis virus, can also help to collapse expanding gypsy moth populations.

Oak Regeneration

We lost thousands of acres of oak saw timber from numerous state game lands in the 2008-2009 outbreak, most notably in Centre County. It’s not all bad news however. Salvage operations once again allowed many of the dead trees to be used by industry to produce important forest products, especially firewood. Since the deer population was in balance in many of these areas, we have also observed very good oak regeneration (in contrast to the results of the 1980s and 1990s) and some of these young forests are now supporting booming populations of grouse, snowshoe hares, and white-tailed deer. As with most things in nature, there are always winners and losers; a warm, dry spring spell can remind us of that.

More information:

Pennsylvania DCNR – Gypsy moth 

Gypsy Moth in North America

By Dave Gustafson and Paul Lupo, Pennsylvania Game Commission Forestry Division

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