ANF Anticipates Noticeable Gypsy Moth Defoliation This Year

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Photo courtesy Kelly Jo

BRADFORD, Pa. – Recreational users on the Allegheny National Forest (ANF) report seeing abundant populations of gypsy moth caterpillars—particularly across the northern half of the Bradford Ranger District.

“The DNCR, Bureau of Forestry has received similar reports about gypsy moth caterpillars defoliating trees on private lands in the region.”, said Cornplanter District Forester Cecile Stelter.

Gypsy moth (Lymantria dispar), a non-native invasive species from Europe, has been in Pennsylvania since 1932 and is notorious for occurring in boom and bust cycles. “When populations are high like they are this year, large quantities of foliage are consumed, and complete defoliation of forested areas may occur. Most healthy trees will grow new leaves in response to the defoliation, but some trees that are already weakened by other stressors, may not withstand severe defoliation.”, said ANF Silviculturist Josh Hanson.

Although oak trees are preferred, gypsy moth caterpillars will feed on numerous other tree and shrub species. Other species on their menu include alder, apple, aspen, basswood, birches (gray, white, and river) boxelder, hawthorn, larch, and witch-hazel.

To limit defoliation, and the negative impacts associated with it, the Forest Service and the DCNR Bureau of Forestry completed aerial suppression treatments across portions of seven ANF recreation areas totaling 2,850 acres. Hooks Brook Boat Access Campground, Tracy Ridge Recreation Area, Handsome Lake Boat Access Campground, Hopewell Boat Access Campground, The Trails at Jakes Rocks, Rimrock Hiking Trail and Overlook, and the Morrison Hiking Trail were all treated with Bacillus thuringiensis kurstaki (Btk)—a naturally occurring soil bacteria and insecticide that is commonly used in organic agriculture. These treatments were part of the Bureau of Forestry’s 2021 gypsy moth suppression program that treated a total of 203,569 acres of state forests, parks, and game lands across Pennsylvania.

Spray treatments are conducted in spring when the caterpillars are small and before extensive defoliation damage occurs.

Two diseases can affect older gypsy moth caterpillar populations and have caused prior outbreaks to collapse. One disease is a nucleopolyhedrosis virus (NPV) which is naturally present in gypsy moth populations and generally causes high mortality only when the gypsy moth populations are at very high densities. The other disease is a fungus called Entomophaga maimaiga is native to Japan and was introduced to the northeastern United States in the early 1990s as a biological control for gypsy moth. Weather plays an important role in determining how effective E. maimaiga will be. Like most fungi, the spores need moisture and high humidity to germinate. Frequent rain and warm temperatures in May and June can contribute to the fungus becoming active during gypsy moth outbreaks. Caterpillars folded in half, like an upside-down “V” are killed by the virus, those hanging head down are killed by the fungus, and caterpillars with their heads up are still alive.

Additional information on gypsy moth biology, the DCNR’s Bureau of Forestry’s 2021 suppression program, and guidance for homeowners and private landowners on gypsy moth management can be found here.