Prionoxystus robiniae (Peck) (Lepidoptera: Cossidae)
Carpenterworms are major pests of hybrid poplars in eastern Oregon and Washington. Larvae burrow into the boles of trees, weakening them and destroying heartwood. The objective of this pest sheet is to convey to professional pest managers a means to monitor moth populations and to suggest a male-trap- out strategy to control the population.
Prionoxystus robiniae is an endemic North American species that is widely distributed throughout US and Canada (Solomon and Hay 1974). Prionoxystus robiniae is not the only Cossidae that attacks poplar. Acossus centerensis (Lintner) is named the “poplar carpenterworm” and can be found from Maine west to Montana, Nevada, Utah, Arizona, and Colorado in the US, and from Quebec west to Alberta in Canada (Carolin 1977; University of Alberta 2016). Another species, Acossus populi (Wik) is called the “aspen carpenterworm” and is found from coast to coast (Baker 1972). Zeuzera pyrina (L.), the leopard moth, is an invasive species from Massachusetts to Pennsylvania that has been in the US since 1909 (Howard and Crittenden 1909) and attacks poplar (Baker 1972).
Two burrowing Lepidoptera pests of poplar have the same species name: robiniae. These are the western poplar clearwing moth, Paranthrene robiniae, and the carpenterworm moth, Prionoxystus robiniae.
Prionoxystus robiniae feeds on a variety of deciduous trees; including oak (Quercus), birch (Betula), ash (Fraxinus), black locust (Robinia), elm (Ulmus), maple (Acer), willow (Salix), cottonwood (Populus), pecan (Carya), and less commonly on fruit trees such as cherry (Prunus), peach (Prunus), apricot
(Prunus), and pear (Pyrus) (Solomon and Hay 1974; Solomon 1988). Prionoxystus robiniae are primarily found in older, often previously damaged trees (USDA 1989).
Carpenterworm moths (Figure 1) have a nationwide range, excluding New England and the northern central Great Plains states.
After hatching, larvae quickly grow and can reach a size of 2.5 cm within a month (Solomon 1967b). Burrowing galleries during larval development are the damaging portion of the life cycle. The larvae bore into the heartwood of the tree, pushing frass and detritus out of the entrance hole. The tunnels are formed in an upward direction and are enlarged steadily as the larva grows. Larvae have a minimum of seven instars, but can go through up to 30 instars (Solomon 1988). Unlike most Lepidoptera, P. robiniae larvae may undergo stationary molts until the correct environmental conditions trigger pupation.
The larvae pupate near the gallery exit, and the pupal exuvia may be found protruding from the exit hole. This species overwinters as larvae in various instars.
Larvae reach full length after seven instars (Solomon 1973). At room temperature (24°C) the pupal period lasts 11–20 days (Forschler and Nordin 1989). In the southern states, one to two years are required for the P. robiniae to go through its life cycle, while in the northern states it requires two to four years (USDA 1989). Sexual dimorphism exists (Figure 2).