Forest Tent Caterpillar Resurgence in Vermont

The numbers of Forest Tent Caterpillar, a native insect that feeds on hardwoods, are on the rise. The Department of Forests, Parks and Recreation (VT FPR) monitors Forest Tent Caterpillar. Trap catches this year increased compared to last year in 12 of 13 sites. Populations seem to be growing across the state. The 2016 aerial survey mapped at least 24,500 acres of FTC defoliation. Heaviest defoliation occurred in Essex, Lamoille, Orleans and Caledonia counties.

Forest Tent Caterpillar (FTC) occurs in hardwood forests in much of North America. It will feed on many species, but prefers sugar maple and ash. Red maple is not a host. 

FTC uses silk to create a trail and pads on host trees where they congregate and rest. Despite their name, they don’t build tents. Tents seen in the cherry and apple trees in the spring are likely the eastern tent caterpillar and tents found in the late summer are probably fall webworm.

The larvae, or caterpillar stage of FTC, are hairy, brownish, with pale blue lines along its sides.  On its back are white spots in the shape of a keyhole or footprint. The larvae hatch from eggs that are laid in barrel shaped masses on small twigs. This emergence is timed to coincide with the unfolding of maple leaves. Larvae eat leaves and as they grow can defoliate trees quickly. After several molts, the larva matures and pupates in a white to pale yellow cocoon that is often inside a rolled leaf. Adult moths are light brown with narrow dark bands on their wings.

Otherwise healthy trees can generally survive several years of defoliation. Damage done by FTC is early in the season and often the tree can refoliate. Other stresses, like this year’s drought can limit the refoliation and compound the stress of defoliation. Defoliation reduces the trees ability to produce and store carbohydrates. As food reserves are depleted, the tree losses its ability to survive the winter, defend itself against secondary pests and maintain its living cells.

Forest tent caterpillars are part of the native hardwood ecosystem. Their populations tend to cycle. The last big infestation spanned 5 or 6 years and peaked in 2006. Natural forces that tend to keep FTC populations in check include birds, parasites, viral and fungal diseases, cold weather and starvation. The friendly fly (Sarcophaga aldrichi) is a native fly that lays its eggs on FTC cocoons.

Forest tent caterpillars are especially of concern to maple syrup producers. By request, FPR will provide assistance in conducting egg mass surveys. In a sugar bush or for backyard trees, if conditions warrant, sprays are available to protect foliage. Technical advice for land managers, sugar bush owners, arborists and home owners is available from VT FPR contact your county forester or VT FPR’s Forest Biology Lab at 802-879-5687.  

Article by Jim Esden, VT Department of Forests, Parks and Recreation.

Photo Credit: William M. Ciesla, Forest Health Management International,