• Latin Name: Prionoxystus robiniae
  • Latin Family Name: Cossidae
  • Common Name: Carpenterworm
  • Other Names: Locust Borer

Origin:
This is a native species in North America and is found throughout the United States and southern Canada and south into Mexico.

Biology:
This small family of moths has about 50 species in North America, with The Carpenterworm the largest. The larvae feed within the solid wood of deciduous trees such as locust, oak, chestnut, poplar, willow, maple, and ash, taking 3-4 years to complete their growth. They also may infest fruit trees such as apricot and pear, causing significant damage due to the large size of the larva as it grows, particularly when the larva is feeding within large branches that may then weaken and fall. Adult moths emerge from the overwintering pupae in early spring, mating and depositing eggs on new host trees. The larvae initially feed in the sapwood but move to the heartwood as they enlarge, often returning to the entrance hole to push out stringy frass that may be seen below the opening. When ready to pupate the larva returns to the hole and the pupa protrudes slightly from it. In some countries the huge larvae are prized as food by native peoples, including the “Witchetty grubs” in Australia and “butterworms” in Chile.

Identification:
The presence of large holes in the trunk or main branches, with frass or sawdust falling from it or around the opening, may indicate an active larva inside. The larvae grow to about ½ inch wide and 3 inches long, and their gallery is usually about 10 inches in length. The adult moths are mottled, lined gray and with a wingspan about 3 inches across. The male’s hindwings are bright orange and brown. The abdomen of this moth is extremely fat and the female is a poor flier.

Characteristics Important to Control:
Infestations of only a few larvae may be killed by physically stabbing them with a sharp wire run into the entrance hole. To monitor the results of this the hole should be lightly plugged to see if it is opened again. There also has been success injecting beneficial nematodes into the entrance hole. This is best done during warm weather with a second application 1 to 2 weeks later increasing the chances of success. Weakened branches may need to be removed.