Forest Tent Caterpillars. Note the keyhole-shaped white spots along their backs (Photo provided — Ronald S. Kelley, Vermont Department of Forests, Parks and Recreation, Bugwood.org)

They hang around on finely spun strands of silky string, blue-black caterpillars parachuting ever so slowly to earth, landing in yards, crawling around on decks and porches – even finding their way into homes.

Over the past few weeks, several people have asked me about them. Some have been coping with large numbers of them, and one person asked if they were the same worms that make their webs in apple trees.

They are not. They are similar, though. Both are hairy. Both are dark colored. Both grow from less than one-eighth of an inch to 2 inches or larger over a six- to eight-week period. Both are tent caterpillars. Beyond that, they’re clearly different.

The caterpillars which, in the spring and early summer commonly make their nests, or tents, in the forks of apple, crabapple and pin cherry tree branches are Eastern tent caterpillars. They’re native to New York state and their damage is pretty much limited to those and other trees of the rose family (e.g. pear, plum, peach, mountain ash) and, for the most part, considered to be only aesthetic. Large nests and defoliation can become particularly unsightly. However, because they are hosted by fruit trees, they are considered orchard pests.

Eastern tent caterpillars’ heads and bodies are black. A single white stripe along the back bordered with reddish-brown is their most distinguishing feature. Along either side, there’s a row of oval blue spots. Short, irregular brownish markings are found on the side of each body segment.

As their name suggests, Eastern tent caterpillars are found throughout the eastern United States, but as far west as the Rocky Mountains. Their northern range includes parts of southeastern Canada.

Unlike the Eastern tent caterpillar, the forest tent caterpillar, which is also a native to New York state, is a defoliator of sugar maple, aspen, oak, ash, birch and many other deciduous forest trees. While they are called tent caterpillars, they don’t construct tents. They spin silken mats on leaves and branches, usually higher-up in the host tree, where they congregate when at rest. Later in their development, they are more commonly found congregating lower in the crown and on the trunk.

Forest tent caterpillars’ heads and bodies are generally a pale-blue black, with fine orange and black lines. Their most distinguishing feature is a single row of distinctly keyhole-shaped white or cream-yellow spots along their backs. They have light blue stripes on their sides, and they’re found in hardwood forests throughout North America.

Reports of forest tent caterpillars outbreaks in the northeast date back to colonial times. And small outbreaks are not at all unusual. More often than not, tree damage is minimal, limited at most to reduced growth and some branch mortality. Unless the trees are already under stress due to disease or environmental factors, new leaves will grow, replacing those that have been eaten.

Occasionally however, forest tent caterpillar populations escalate beyond tolerable levels and defoliation devastates expanses of forestland. That’s exactly what happened last year in a number of locations across the North Country and Vermont, where sporadic areas of heavy infestation resulted in some maple syrup producers reporting defoliation in their sugarbushes (stands of sugar maple trees) of 20 to 90 percent. Defoliation of sugar maples may result in reduced sap flow and sugar content the following year.

Paul Hetzler, a Natural Resources Educator with Cornell Cooperative Extension of St. Lawrence County reported earlier this year that the vast majority of those defoliated maples did not grow a second set of leaves, possibly due to stress from drought during the previous summer. Hetzler went on to state that some of the affected maples on south-facing slopes started to put on a flush of new leaves in October, before the partially-developed leaves succumbed to a freeze.

According to New York state Extension Maple Specialist Stephen Childs, “There are many pesticides labeled for treatment of sugar maples for forest tent caterpillar,” but he can find “none that have established tolerances for maple sap or maple syrup.” Childs goes on to say that “unless they are exempt from a tolerance by the Environmental Protection Agency, they should not be used in a forest where sap is harvested.”

Bt (Bacillus thuringiensis; a soil-dwelling bacterium, commonly used as a biological pesticide) products are exempt. So, “a producer would need to use a Bt product that had forest, maple or deciduous trees and tent caterpillar and aerial application on the label.” He adds that, “neem extract materials are also tolerance exempt.”