Description

Prionoxystus robiniae, the carpenterworm moth or locust borer moth, is a large wood feeding moth species in the order Lepidoptera, family Cossidae. Adult carpenterworm moths are dark brown and black with large, tapering, gray bodies with white speckles. The forewings possess bold black and gray mottling and are held rooflike over the abdomen when at rest. Hindwings of males are yellowish orange in the rear two-thirds. Hindwings of females are black and gray. Adult wingspan ranges from 43 mm to 85 mm. Females are larger than males. The head is small and partially tucked under the thorax. The antennae are black, slender and lined with short hairs. No antennal club or hook is present.

Carpenterworm larvae are large, hairless, yellowish pink caterpillars that can reach three inches in length when fully mature. Larvae are rarely encountered since they tunnel through solid wood. Eggs are about 1 mm in diameter and dark green in color. Pupae are similar in size to adults and are brown and spiny. They are sometimes encountered protruding from exit tunnels in host trees.

Two species within the genus Prionoxystus are recorded from Louisiana. The carpenterworm is by far the more common, but a second species, the little carpenterworm (Prionoxystus macmurtrei), also has been documented. The latter species is smaller than the carpenterworm and possesses translucent wings. The species appears to be more common in adjacent states, but this is likely due to insufficient collecting or misidentifications in Louisiana.

Life Cycle

Prionoxystus robiniae is sometimes called the locust borer because carpenterworm larvae bore into the wood of living locust, oak, willow, maple and ash trees. Carpenterworm moths are holometabolous, which means they go through complete metamorphosis, as are all Lepidoptera. Development involves four stages: egg, larva, pupa and adult. The life cycle requires three to four years to complete. After mating during the summer, adult females lay up to 200 eggs each in small clumps on the bark of host trees during fall. After several weeks, larvae hatch and begin boring into the wood of the host tree. During the next two to three years, larvae consume wood, create tunnels and grow. Young larvae tunnel under bark or outer layers of wood. Older larvae tunnel extensively in deeper layers of the tree. Depending on environmental conditions such as humidity and heat, larvae may undergo eight to 31 molts (instars). Once the larvae have finished their final molts, they begin pupation. Pupation occurs near an exit hole prepared by the larva. Adults exit the tree after emerging from their pupal cases. Some evidence exists that female adults attract males using pheromones. Adults do not feed and are short lived.

Ecological Significance and Pest Status

The carpenterworm moth is found throughout the U.S. and southern Canada, with many more records in deciduous forests in the eastern half of the U.S. Populations are spotty in the western half of the U.S. Heavy infestations of larvae can kill trees, but this is relatively uncommon and usually occurs in overmature or stressed trees. Lumber produced from infested trees may be devalued due to holes, openings and tunnels. In most cases, basic tree care and irrigation of high value trees can prevent carpenterworm larvae from causing excessive damage.

Carpenterworm larvae feed exclusively on woody tissue, which is mostly cellulose. Insects are incapable of digesting cellulose without the aid of a diversity of symbiotic microorganisms in their digestive organs, mainly bacteria and protists. These relationships have been extensively studied in termites and other wood feeding insects but have been rarely studied in wood feeding Lepidoptera such as the carpenterworm. This is a fertile area of research and discovery for insect physiologists.

References

Brou Jr., V.A. 2009. Prionoxystus robiniae (Peck, 1818) (Lepidoptera: Cossidae) in Louisiana. Southern Lepidoptera News 31: 124-125.

Brou Jr., V.A., R. Patterson and R. Tyler 2023. Prionoxystus macmurtrei (Boisduval,1829)

(Lepidoptera: Cossidae) in Louisiana. Southern Lepidoptera News 45: 69-70.

BugGuide. 2025. Carpenterworm Moth - Hodges#2693 (Prionoxystus robiniae). (accessed 28 April 2025).

Frank, S., J. Baker, and S. Bambara. 2001. Carpenterworm. North Carolina Cooperative Extension Service. (accessed 28 April 2025).

Geisel, P. M. 2021. Pest Notes: Carpenterworm. University of California Agriculture and Natural Resources. (accessed 28 April 2025).

Schoorl, J.W. (Pirn), Jr. A phylogenetic study on Cossidae (Lepidoptera: Ditrysia) based on external morphology. Zoologische Verhandelingen 263, 28.xii.1990:1-295.

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