**Forest Tent Caterpillars** (*Malacosoma disstria*) are native North American moths whose larval stage is infamous for causing large-scale, widespread **defoliation** of hardwood forests. Unlike the Eastern Tent Caterpillar, this species does **not** build a large, prominent silken tent. Instead, the caterpillars create a silken **mat** on the trunk or branches where they congregate during rests and molts. While defoliation can be extensive, healthy hardwood trees (e.g., aspen, oak, maple) usually tolerate repeated attacks, re-foliating within weeks.
Taxonomy and Classification
Forest Tent Caterpillars belong to the family Lasiocampidae (lappet moths) in the order Lepidoptera. They undergo complete metamorphosis. They are closely related to the Eastern Tent Caterpillar but are distinct in their social behavior and host preference. They prefer broadleaf trees, especially trembling aspen, sugar maple, and oaks.
Physical Description
The **adult moth** is stocky and dull orange-brown with two distinct parallel, oblique dark lines running across the forewings. It is nocturnal.
The **larvae (caterpillars)** are the destructive stage, growing up to $2$ inches long.
- **Coloration:** Blue-gray to brown with fine orange lines along the sides and a key distinguishing feature: a row of white, **keyhole- or footprint-shaped spots** running down the middle of the back.
- **Eggs:** Laid in a distinctive, dark brown, varnish-like **band** that completely encircles a small twig.
Distribution and Habitat
Forest Tent Caterpillars are native and widely distributed across the hardwood and aspen-dominated forests of Canada and the United States. Their habitat is the foliage and bark of deciduous trees. They are cyclical pests, with major outbreaks occurring roughly every 10–15 years, lasting 2–4 years in any given location.
Behavior and Damage
The moth has one generation per year. It overwinters as a fully formed larva encased in an egg mass on a twig. The larvae hatch in early spring (timed with bud break) and immediately begin colonial feeding, moving up the tree trunk and branches in large, dense groups (hence “processionary”).
The damage is primarily **defoliation**:
- **Cosmetic Impact:** Defoliation during an outbreak can strip entire tracts of forest, creating an aesthetic nuisance and concern.
- **Low Health Impact:** Healthy trees usually survive, as the defoliation happens early, allowing the tree to produce a second flush of leaves (re-foliate) and recover.
- **Stress:** Repeated severe defoliation over 3–4 years, or attack on stressed/diseased trees, can lead to branch dieback or mortality.
Management and Prevention
Control is often unnecessary for large, healthy forests, but measures are taken for yard trees and high-value stands.
- **Egg Mass Removal:** In winter, inspect and prune out the distinct, dark egg masses encircling twigs.
- **Banding:** Apply a sticky barrier or band around the trunk of trees in spring to trap the crawling caterpillars before they reach the foliage.
- **Biological Insecticides:** Apply **_Bacillus thuringiensis_ (Bt)** to the foliage when the caterpillars are small (early spring). Bt is highly effective and safe, killing the caterpillars only after they ingest the treated leaf tissue.
- **Monitoring:** Outbreaks usually collapse naturally after a few years due to parasitism and viral disease.
Conservation and Research
Forest Tent Caterpillars are managed as a cyclical forest pest. Research focuses on predicting outbreak cycles and understanding the role of natural enemies (especially parasitic flies) in their population collapse.