As I mentioned in ‘2nd Night of Moth Week ‘, I find these moths frequently when running lights amongst the cottonwoods that line the shores of the Columbia River. These big members of the family Cossidae are rather handsome, though not exactly pretty. They and their cousins Prionoxystus robinae don’t seem to be agile flyers, although they are fast, and they often blunder into things in flight, like the head of a recreational naturalist bent over trying to photograph bugs, and they are big enough that it’s rather startling when one smacks you in the temple.
I’ve never seen an Acossus populi on the bark of a cottonwood, but I’ve seen enough cottonwood bark to guess that all of that crosshatching on their wings makes for very good camouflage, and probably protects them well as they are ovipositing on those trees. Once the eggs hatch the larvae begin boring into the tree. I cannot ascertain whether they eat the wood or the cambium, but they aren’t really considered to be a pest species so they must not do much damage.
Description– “A rather large (5.0-6.8 cm wingspan) heavy-bodied light grey moth. The thorax is grey, with a narrow white band bordered by black at the anterior and posterior edges, and the abdomen is dull grey. Forewings are dirty white, crossed by a network of fine dark grey broken lines. Two or more of these lines in the median area are usually darker, more prominent and often partly joined. The fringe is checkered grey and black, with black scales marking the veins. Hindwings are grey with a net of fine black lines as in the forewings. The antennae are narrowly bipectinate in males and serrate in females.” Acossus populi – University of Alberta Museums Search Site
Similar species– “The similar Poplar Carpenterworm (A. centerensis) has two-toned forewings with the basal half much darker, and poorly marked white hindwings. Females of the Carpenterworm (Prionoxystus robiniae) are similar, but have grey blotches on the forewings, and males have large yellow-orange splotches in the anal area of the hindwings.” Acossus populi – University of Alberta Museums Search Site
Habitat-Along rivers and wetlands, and in montane areas- Wherever their hosts grow.
Range– Western and northeastern North America; possibly region wide in the PNW, but apparently undersurveyed.
Eats– Larvae feed on the wood of Populus spp., e.g., cottonwoods, aspen, and poplar.
Eaten by– Presumably a host for parasitoids in Hymenoptera and Diptera, and probably preyed upon by insectivores of all classes, but I can find nothing specific for this species.
Adults active– Nocturnal; adults fly in June, July, and August
Life cycle– “There is little information available on the life history of the Aspen Carpenterworm. Like our other carpenterworm species, the larvae are borers and live in galleries they create in the trunks and stems of poplar trees. The closely related Carpenterworm takes three to four years to complete the life cycle, and it is likely that the Aspen Carpenterworm also takes several years to mature. Adults are usually collected at light. Although the larval burrows can damage the host trees, they are considered to be of no economic concern.” Acossus populi – University of Alberta Museums Search Site
Etymology of names– Acossus is probably from the Latin words for ‘away from grubs living under bark’ to indicate that this is similar to, but different from, other carpenterworm moths. The specific epithet populi refers to their using members of Populus as larval hosts.