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Forest Tent Caterpillar

A Technical Report from The Bartlett Tree Research Laboratories

Identification, Biology, and Management

The forest tent caterpillar, Malacosoma disstria, is a native pest to North America and is found throughout the United States and Canada, from coast to coast. This caterpillar is mainly considered a forest pest but will cause defoliation on deciduous species in the landscape.

Egg mass

Appearance and life cycle
The forest tent caterpillar is black when it first hatches and gradually develops distinct coloration during the course of 5 instars. The larvae generally appear at the same time as leaves begin to expand. The older larvae are easily identified by their blue coloration with white key-hole shaped spots along their backs. Forest tent caterpillars do not actually produce tents, but may spin a nest of fine threads against the trunk of a tree where the larvae congregate at night. Pupation occurs over a 7-10 day period in a cocoon that consists of one or more leaves folded and held together by silk threads. Egg masses are fairly conspicuous black bands of up to 350 eggs, laid on small twigs. There is one generation per year.

Damage begins with open feeding in the spring as leaves expand. The most common hosts are in the genera Populus, Salix, Alnus, Betula, Prunus, and Quercus, although many broadleaved deciduous plants may be fed upon. Tattered appearance of leaves followed by heavy defoliation is a symptom of heavy infestations. In natural situations, heavy infestations can last 3-6 years before natural parasitoids regain control over population levels. The most common damage from infestations is a reduction in growth with some branch kill. Trees will normally replace foliage with a second flush, but repeated defoliation can weaken the tree and lead to secondary disease and insect issues.

Partial control can be achieved using cultural methods. Removal of egg masses in the winter is a good way to reduce damaging populations in the spring. The egg masses can be removed anytime after they are laid, but are most conspicuous when there is no foliage in the winter. Masses of feeding larvae can be removed by hand or washed off of foliage with a strong stream of water. Naturally-derived chemical control methods using Spinosad (Conserve) or Bt-based products are extremely effective and reduce the need for removal of egg masses, which might be located high in a tree. Fertilization based on soil analysis will help defoliated trees to replace damaged or lost tissue.

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