**Goat Moths** (genus *Cossus*, primarily the **European Goat Moth** (*Cossus cossus*) and the **Carpenterworm** or **American Goat Moth** (*Prionoxystus robiniae*)) are large, destructive moths whose larvae (caterpillars) are major pests of hardwood trees and timber. The caterpillar spends several years boring deep tunnels (**galleries**) into the heartwood and sapwood of living trees, leading to structural weakness, wood decay, and reduced timber value. The name “Goat Moth” is attributed to the strong, unpleasant, goat-like odor emitted by the larvae.
Taxonomy and Classification
Goat Moths belong to the family Cossidae (carpenter moths) in the order Lepidoptera. They undergo complete metamorphosis. They are distinct due to their long larval stage, which can last from one to four years, and their exclusive habit of boring into wood, making them some of the longest-lived moth larvae.
Physical Description
The **adult moth** is large, stocky, and cryptically patterned, with dull gray-brown or mottled wings that provide camouflage against tree bark. It is nocturnal and rarely seen.
The **larvae (caterpillars)** are the destructive stage:
- **Appearance:** Large, growing up to $3$ or $4$ inches long. Often reddish or pinkish on the back and yellowish below, with a prominent dark head.
- **Odor:** Possesses a defensive, pungent, musky odor likened to goats, which is often detectable near infested trees.
- **Damage Sign:** Large entrance holes or tunnels on the trunk, often oozing **wet sawdust** (**frass**) mixed with sap and larval excrement. This frass is pushed out of the tunnels as the larva bores deeper.
Distribution and Habitat
Goat Moths are distributed across temperate regions of the Northern Hemisphere, infesting Europe, Asia, and North America. Their habitat is the trunks and large limbs of various hardwood trees, including oak, willow, ash, maple, and fruit trees. They prefer weak, stressed, or older trees for egg-laying.
Behavior and Damage
The female moth lays eggs in bark crevices or near existing wounds on the lower trunk. Upon hatching, the young larvae bore into the bark and eventually penetrate the heartwood, creating extensive, deep tunnels that weaken the structure of the tree.
The damage is severe and long-term:
- **Structural Weakness:** Tunnels compromise the trunk’s integrity, making the tree susceptible to wind damage and breakage.
- **Wood Decay:** The tunneling provides entry points for wood-rotting fungi and bacteria, leading to irreversible internal decay.
- **Physiological Stress:** Galleries disrupt the flow of water and nutrients, causing branch dieback and overall decline in tree health.
- **Frass Accumulation:** The expulsion of frass, often wet and brown, is the most obvious sign of an active, severe infestation.
Management and Prevention
Control is challenging because the pest is protected inside the wood for most of its life.
- **Mechanical Removal:** For high-value trees, visible larvae can sometimes be removed using a wire inserted into the tunnel.
- **Injectable Insecticides:** Small amounts of specialized systemic or contact insecticides can be injected directly into the active bore holes to kill the larva inside, followed by sealing the hole. This requires professional expertise.
- **Exclusion/Protection:** Keep trees healthy and avoid wounding the bark, as the female moths are attracted to damaged areas for egg-laying.
- **Removal:** Heavily infested, weak trees should be removed and destroyed to prevent the moth from completing its life cycle and spreading to other trees.
Conservation and Research
Goat Moths are managed as serious forest and orchard pests. Research focuses on developing effective pheromone traps to monitor adult flight timing and investigating biological controls, such as parasitic nematodes that can penetrate the larval tunnels.