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Your first hint that something bad is about to happen may be the thick spider web you see wrapped around the fork of a tree branch. Then you notice a few others. A week or two later, it's clear that something is eating the leaves off your trees.

Spiders? No, they're eastern tent caterpillars. They are a particular pest of trees from the rose family, such as wild cherry, apple, and crabapple, but will also infest plum, peach, and cherry, and to a lesser degree ash, birch, willow, maple, oak, poplar, blackgum, redgum, and witch-hazel.

Common in the United States east of the Rockies, their populations are normally kept in check by natural predators, but it seems that large eruptions occur about every 10 years.

Unchecked, these caterpillars can completely defoliate a tree, which won't kill the tree, but it can certainly ruin your landscape or your fruit crop.

IDENTIFICATION

The most prominent indicator of the Eastern tent caterpillar is the one- to two-foot long silky web they spin in the crotches of tree branches. You can usually see masses of them sunning themselves on the web in the late morning.

Don't confuse these nests with nests of the fall webworm. The webworm's nest is at the tips of branches and encloses foliage. If you see this type of web, you're dealing with different critter.

The hairy caterpillars are mainly black, with tan bristles, a white stripe down the back, and blue and white markings along their sides. Very tiny when they hatch, they grow to about two inches long.

HABIT

The adult moth lays 200 to 300 eggs in a single cluster in the fall. The three-quarter-inch long egg masses wrapped all around pencil-sized twigs look foamy and shiny.

The eggs hatch just as the buds start forming on the trees. The caterpillars in the brood will remain together as a colony, building a communal nest for protection from predators, cold, and rain. As they feed, they expand the nest in layers.

Typically, the caterpillars leave the nest in mid morning, mid afternoon, and early evening to feed. At other times, they are sunning themselves on the nest, working on it, or sheltering inside it.

After 4 to 6 weeks, the caterpillars wander off individually to find a place to build their white or yellowish cocoons. The adult moths emerge in about 2 to 4 weeks. They breed only one generation per year.

PREVENTION

Inspect trees in the winter for egg masses and remove affected twigs.

CONTROL

For small infestations, consider letting natural predators (birds and parasitic wasps) have their way. The tree may be unsightly, but it won't suffer lasting harm.

You can remove the nests using a stick to wind up the silky web and its inhabitants, dropping them into a bucket of soapy water. It's best to do this early morning before they leave the nest to feed, or midday when they've returned to the nest after feeding to sun themselves.

Sprays incorporating the natural bacterial agent Bt are also effective, as are sprays of carbaryl, malathion, or methoxychlor. Always follow directions when using insecticides.

Since the caterpillars are somewhat protected from chemical or biological insecticides while in their nests, it's best to spray when they're out of the nest feeding or congregated on the nest midday. Insecticides are more effective when the caterpillars are younger, so prompt treatment is important.

Learn more about this author, Steve Holder.
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