This is one of the beetles I mentioned finding in 3rd Night of Moth Week. I’ve probably seen Centrodera spurca before, most notably running lights up on the Wind River in Skamania County, Washington, but I was never able to ascertain for sure that it was C. spurca, rather than C. dayi. This time I was able to get good, closeup photos, because this was an amazingly calm beetle that mostly just stayed motionless in one spot, as well as having an internet connection with which to determine the salient details, and I feel very good about this identification.
This species of longhorn beetle has been saddled with the common name yellow Douglas-fir borer, but I did not use it in the title because the larvae are not by any means confined to Doug-fir (and in fact don’t even show a preference for that tree), and they are not borers into the wood, but instead consume dying and decaying roots. This is yet another reason I prefer the specificity of scientific names, even though they are often obscure and sometime themselves promulgate misinformation about a species, but are at least quite specific to the species, regardless of one’s region or language.
Description– Large (19-30mm) yellowish brown longhorn beetle with a single dark spot near the edge in the middle of each elytron, and a spine on each margin of the pronotum; elytral apex at the suture has a single triangular ‘tooth’; legs, antennae, and head are the same yellowish brown; hairs near the front of the elytra are flattened to appressed; final four antennal segments are smooth with the hairs strongly appressed;
Similar species– Centrodera dayi has fuzzier final antennal segments, sub-erect hairs on front of elytra, lacks tooth at apex of elytra; Ortholeptura sp. have stripes and/or multiple spots on elytra.
Habitat– Forested areas and woodlands.
Range– Western North America; found region wide in the PNW in appropriate habitat.
Adults active– Nocturnal; May through August
Eats– “the larvae of Centrodera spurca…commonly feed in rotting stumps and roots (and possibly on living roots) of several kinds of trees and shrubs…” ser.4:v.32 (1962-1968) – Proceedings of the California Academy of Sciences, 4th series – Biodiversity Heritage Library ; these seem to include Douglas-fir, Garry oak, Western serviceberry, madrone, ceanothus, and probably quite a few other species.
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.
Life cycle– “The finding of two sizes of larvae and an adult at the same site in April, at Mill Valley, and an adult in its pupal cell in October, makes it fairly sure that the life cycle takes at least two years. Some mature larvae must form their pupal cells, pupate, and tranform into adults, in the fall. However, the fact that in the San Francisco Bay area of California the species is on the wing over a period of four months (table I) suggests that some larvae may not pupate until the spring…they wander freely through the soil, and pupate in the wood or in the soil as it suits them.” ser.4:v.32 (1962-1968) – Proceedings of the California Academy of Sciences, 4th series – Biodiversity Heritage Library
Etymology of names– Centrodera seems to be from the Greek words for ‘point’ and ‘neck’, and possibly refers to the spine on each side of the pronotum, but I cannot verify this. The specific epithet spurca is from the Latin word for ‘unclean’, and may refer to the dingy coloration, but I can’t verify this either.