Carpenter moths, also known as carpenterworms, are wood-boring insects whose larvae damage trees and wooden structures. Unlike many wood-boring pests, which are beetle larvae, carpenterworms are caterpillars in the moth family Cossidae. Found across the United States and southern Canada, they primarily affect hardwoods like oaks, maples, and willows, as well as fruit and ornamental trees. Their larvae tunnel deep into wood, causing structural weakening and a decline in tree health.
Identifying Carpenter Moths
Adult carpenter moths are large, robust insects with a wingspan of 1.7 to 3.3 inches (43 to 85 mm). Their forewings are mottled gray and black, providing camouflage against tree bark. Males are smaller than females, usually two-thirds their size, with distinctive yellowish-orange hindwings, while females have off-white or light gray hindwings.
The larval stage, or caterpillar, is large, reaching lengths of 2 to 3 inches (5 to 7.5 cm) when fully grown. Larvae are fleshy, greenish-white or pinkish, with a dark brown head. Unlike wood-boring beetle larvae, carpenterworm caterpillars have prolegs, leg-like appendages with bands of hooks.
The Carpenter Moth Life Cycle
The carpenter moth life cycle begins when adult females lay eggs in groups of 200 to 1,000, often in bark crevices, under lichens, or near tree wounds. Eggs hatch within one to two weeks. The tiny, reddish-pink larvae immediately bore into the tree, either through bark or existing holes.
Young larvae initially feed in the inner bark and sapwood, creating small tunnels. As they grow, tunnels expand to 0.5 inches (1.27 cm) in diameter and 6 to 10 inches (15 to 25 cm) in length, primarily running vertically within the heartwood. The larval stage is long, lasting two to four years, depending on location and climate.
During this period, larvae continuously feed on wood, periodically returning to the gallery entrance to consume callus tissues, cambium, and inner bark. Pupation occurs in late spring, lasting two to three weeks, at the upper end of the larval tunnel near the exit hole. When ready to emerge, the pupa wriggles to the burrow mouth, with the empty pupal case often protruding from the exit hole after adult moths emerge.
Recognizing Infestation and Damage
Identifying a carpenter moth infestation involves specific signs on affected trees. A common indicator is frass: sawdust-like droppings expelled by larvae from their tunnels. This material, a mixture of wood particles and larval excrement, is found in bark crevices or at the base of infested trees.
Dark sap spots, discolored and moist areas on the tree trunk, also signal early infestation as larvae feed within the sapwood. As larvae mature and enlarge galleries, visible tunnels or holes appear on the bark, often in crotch areas or existing cracks. In older infestations, empty pupal cases might protrude about two-thirds out of the tree bark, indicating adult moths have emerged. Extensive tunneling weakens branches and stems, leading to branch dieback, reduced growth, and increased breakage risk, especially in high winds.
Controlling Carpenter Moth Problems
Managing carpenter moth infestations involves preventative measures and direct control methods. Maintaining tree health and vigor is a primary preventative strategy, as stressed or injured trees are more susceptible to attack. This includes proper irrigation and avoiding mechanical bark injuries, which provide entry points for egg-laying females.
For existing infestations, mechanical control is effective for light cases, involving probing tunnels with a wire to puncture and kill larvae. If only a few trees are affected, judicious pruning and removal of heavily infested wood helps slow insect spread. Chemical control options are also available, primarily targeting emerging adults or young larvae.
Preventative insecticide treatments, such as carbaryl or permethrin, apply to tree trunks and larger branches in mid-May, before adult moths lay eggs. Once larvae are inside wood, they are more difficult to control; however, injections of insect-attacking nematodes like Steinernema carpocapsae or Steinernema feltiae into tunnels have shown effectiveness. For widespread or severe infestations, professional pest control services may be necessary, as they have access to a broader range of products and specialized application equipment.