**Inchworms** is the common name given to the larvae (caterpillars) of moths in the family Geometridae. They are easily recognizable by their unique, looping method of locomotion, caused by having very few prolegs (fleshy abdominal legs) in the middle segments of the body. They are common defoliating pests of many forest, ornamental, and fruit trees, including oaks, maples, apples, and various shrubs. They can cause severe aesthetic damage and, in outbreak years, significant stress to forest trees.
Taxonomy and Classification
Inchworms belong to the order Lepidoptera (moths and butterflies), family Geometridae. They undergo complete metamorphosis. The name Geometridae literally means “earth measurer,” referring to the inchworm’s characteristic “looping” or “measuring” walk. They are also sometimes called **loopers**. The lack of mid-abdominal prolegs is an evolutionary adaptation that dictates their movement.
Physical Description
Inchworms are slender, smooth caterpillars, typically 1 to 1.5 inches long.
- **Appearance:** Usually green, brown, or gray, often mottled to perfectly mimic a twig or leaf stem (excellent **camouflage**).
- **Movement (Key ID):** To move, they anchor their rear prolegs, extend their body forward, anchor their front legs, and pull the rear end up to meet the front, creating a distinctive loop.
- **Adult Moths:** Generally small, cryptically colored moths, though some species, like the cankerworms, have wingless females.
- **Defense:** When disturbed, inchworms often stop moving and hold their bodies rigid at an angle from the plant stem, further mimicking a twig.
- **Damage Sign:** Chewed leaves, often starting with small holes and progressing to complete defoliation during heavy feeding periods.
Distribution and Habitat
Inchworms are found globally wherever trees and shrubs grow. Their habitat is the foliage of host trees, primarily feeding on the soft, new leaves in spring and early summer. They are a common sight, dropping down from trees on silk threads when disturbed.
Behavior and Conflict
Inchworms are generalist defoliators, and large outbreaks can be significant.
- **Feeding Period:** They are most damaging in the spring when trees are putting out new, tender leaves. A healthy, mature tree can usually withstand a single defoliation, but repeated attacks over several years can weaken or kill it.
- **Nuisance:** During heavy outbreaks, the sheer number of larvae and the silk threads they use to drop from trees can be a severe nuisance in parks and yards.
- **Stress:** The loss of foliage stresses trees, making them more vulnerable to secondary pests, diseases, and drought.
Management and Prevention
Control is integrated pest management (IPM), targeting the young larvae in spring.
- **Monitoring:** Inspect host trees visually for small larvae and defoliation symptoms in early spring.
- **Biological Control (Key):** Apply **_Bacillus thuringiensis_ var. *kurstaki* (Btk)** to the foliage when the larvae are small. Btk is highly effective against young caterpillars and safe for beneficial insects.
- **Banding (For Wingless Moths):** For species like Cankerworms where the female moth is wingless, apply a sticky band (like Tanglefoot) around the tree trunk in late fall or early spring to trap the females as they attempt to climb the trunk to lay eggs.
- **Natural Enemies:** Inchworm populations are often naturally controlled by parasitic wasps, parasitic flies, and predatory birds.
Conservation and Research
Inchworms are managed as forest and ornamental pests. Research focuses on understanding the cyclical nature of their outbreaks and improving the application timing of biological controls to maximize efficacy while minimizing impact on the ecosystem.