The Eastern Tent Caterpillar (Malacosoma americanum) is a familiar springtime pest across North America, known for its distinctive silken nests that appear in the forks of trees. This insect primarily targets ornamental and fruit trees, preferring black cherry, apple, and crabapple varieties. While a single season of feeding rarely kills a mature tree, defoliation can be unsightly, and repeated infestations may weaken the host. Managing this pest relies on accurate identification and applying the appropriate control method at the correct stage of the caterpillar’s life cycle.
Identifying the Eastern Tent Caterpillar and Its Cycle
The Eastern Tent Caterpillar is easily recognized by its physical appearance and the location of its communal nest. The larva is a hairy, brownish-black caterpillar featuring a prominent white stripe down the center of its back. Along the sides of this stripe are thin yellow lines and a series of distinct oval blue spots, growing to about two inches in length when fully mature.
The silken tent, which houses the colony, is constructed in the crotch or fork of branches, appearing in early spring as soon as the tree’s leaves emerge. This location differentiates it from the Fall Webworm, which builds nests covering the ends of branches later in the year. The caterpillars hatch from their overwintering egg masses around March or April and remain actively feeding for approximately four to six weeks. They use the tent for protection from predators and temperature extremes, leaving the structure to feed on nearby foliage, often in the evening or early morning.
Non-Chemical Removal Methods
Physical removal offers an immediate, low-impact solution for managing infestations, particularly on smaller or more accessible trees. The most effective time to destroy the nest is during the early morning or evening, or on cool, rainy days, when the majority of the caterpillars are gathered inside the protective silk.
If the nest is on a small, expendable branch, use pruning shears to cut off the entire section and dispose of it immediately. For larger nests on primary limbs, the tent can be removed by winding the silk around a long stick or pole, collecting the entire mass of caterpillars. The collected nests and caterpillars should be dropped into a container of soapy water or sealed in a bag and discarded.
A strong jet of water from a garden hose can sometimes dislodge smaller, newly formed tents, though this is less reliable than manual action. Avoid attempting to burn the tents out of the tree. Using a torch or open flame inflicts severe and permanent damage to the tree’s bark and underlying tissue, which is often worse than the insect damage, and carries a high risk of accidental fire.
Targeted Biological and Chemical Treatments
For widespread or inaccessible infestations, such as those high in large trees, targeted treatments are necessary. The preferred biological control agent is Bacillus thuringiensis var. kurstaki (Btk), a naturally occurring soil bacterium. Btk is a selective insecticide that must be ingested by the caterpillar to be effective, paralyzing its digestive system within hours.
Btk is highly effective only against young caterpillars and must be applied while they are actively feeding on the foliage outside the tent. It is considered safe for beneficial insects, including pollinators, birds, and mammals.
Neem oil is another option, acting as both an insecticide and a growth regulator, but requires careful application to avoid potential leaf burn on hot, sunny days. Chemical insecticides can be used for severe cases, but they are most effective when applied directly to the feeding caterpillars, not just the tent itself. Since mature caterpillars are more resistant to chemical control and have already caused most defoliation, treatment is best timed shortly after the eggs hatch in early spring.
Future Prevention of Infestation
Effective long-term control focuses on preventing the problem before the caterpillars hatch in the spring. The most proactive measure is the removal of overwintering egg masses during the dormant season, from late fall through early spring, when the tree branches are bare.
Eastern Tent Caterpillar egg masses appear as dark, varnished-looking, foam-like collars that completely encircle small twigs. Each mass can contain hundreds of eggs, and removing just one significantly reduces the potential infestation for the following season. The affected twig should be pruned off and disposed of, or the egg mass can be scraped away from the branch into a bucket of soapy water.
Maintaining the overall health of the host tree through proper watering and fertilization helps it withstand minor feeding damage and recover more quickly from defoliation.