Forest Tent Caterpillar and other Caterpillars

Problem type: Insect

Name of problem:
 Forest Tent Caterpillar and other Caterpillars

Plant name(s): Oak, ash, elm, maple, apple, basswood, poplar and other hardwoods

Symptoms / Characteristics:
The forest test caterpillar in one of the major defoliators of deciduous trees. Leaves are completely stripped with only the major leaf veins and petioles remaining. Many caterpillars feed by chewing entire portions of the leaves. Other leaf feeding insects may only chew through a portion of the leaf, leaving the leaf papery. These include leaf miners and skeletonizers.

Forest tent caterpillars lay their eggs in masses completely encircling small twigs. Eggs are dark brown in color and are covered with a glue-like substance making them appear glossy. Other caterpillars may lay eggs on the underside of leaves, twigs or almost anywhere on the host plant. Most caterpillars overwinter as eggs, hatching around the same time that leaves are unfolding. Others might overwinter as adults, laying eggs first thing in the spring. The forest tent caterpillar has only one generation per year with population levels being cyclical. Most outbreaks last for 3 years causing severe damage, followed by a decline in population, resulting in minimal damage.

Caterpillars come in an array of colors, sizes and appearances. Forest tent caterpillars are among the easiest to identify. Mature larvae are pale blue and have distinctive keyhole-shaped white spots along their backs. A close relative, the eastern tent caterpillar has a single solid stripe down its back. When forest tent caterpillars are not feeding, they can be found in a large mass on trunks or larger branches, especially during inclement weather. Mature larvae spin silken cocoons to pupate, often rolled within a leaf. Adults then emerge in July and lay eggs. Many moth and butterfly caterpillars find places to hang to form a chrysalis, emerging 1 to 2 weeks later as adult moths or butterflies.

Defoliation of the tree caused by the larvae usually has little effect on tree health. Two or more successive years of defoliation may cause branch and twig dieback and reduced growth. Very rarely is there tree mortality due to defoliation, as new leaves are usually formed. Repeat defoliations along with other stress factors may open the tree to other diseases or pests.

Control / Preventions:
Egg masses found on smaller trees can be removed and destroyed. Masses of larvae found on trunks and branches can be knocked off and killed. A biological control using the bacterium, Bacillus thuringiensis (Bt), and several insecticides are registered for use against caterpillars.

There are several species of flies and wasps that feed on eggs, larvae and pupae of most caterpillars. Some fly species may become abundant during an outbreak of the forest tent caterpillar. Predators such as beetles, ants, spiders and birds also feed on caterpillars.