According to the calendar it’s still autumn; however, in practical terms winter has settled in across much of the U.S. For those of us who study wood-boring beetles in the families Buprestidae (jewel beetles) and Cerambycidae (longhorned beetles), our time for collecting ended long ago. Adults of most species are active in spring and early summer, although some species don’t really make their appearance until summer is in full swing and a few rather distinctive species in genera such as Crossidius and Megacyllene make their appearance exclusively during fall. There is one longhorned beetle, however, that can actually be encountered in its greatest numbers during the dead of winter—Rhagium inquisitor, or the “ribbed pine borer.”
Rhagium inquisitor is unique among North American cerambycids in several respects. Most species in the family overwinter as mature or immature larvae, the former triggered to pupation by the first warm days of late winter and early spring in preparation for emergence as adults a few weeks later. Rhagium inquisitor, on the other hand, pupates during late summer and fall and then transforms to the adult before winter sets in (Linsley & Chemsak 1972), passing the winter in this stage and emerging during the earliest days of spring. Also unique among North American cerambycids is the place of pupation—directly under the bark. This contrasts with most other species, which either feed and pupate within the sapwood or feed under the bark but then bore into the sapwood for pupation. The species breeds exclusively in the trunks of dead conifers, with pines (Pinus spp.) especially favored, and as a result one can easily encounter the adults by peeling back the bark of dead pines during winter. Pupation takes place within distinctive rings of frass and coarse, fibrous wood shavings, prepared by the larva prior to pupation, so even when adults and larvae are not present the occurrence of this species can be determined by the occurrence of their pupation rings.
Not only are the overwintering and pupation habits of this species unique, but the adults themselves are distinctive from all other North American cerambycids (Yanega 1996) in their appearance—”big-shouldered” build, heavily “ribbed” elytra, and unusually short antennae (that are anything but “longhorned”). Lastly, the species is distributed not only in the boreal forests of North America, but Europe and Asia as well. The species is extremely variable in size and sculpturing, which combined with its Holarctic distribution has led to an unusually high number of synonyms. In fact, much of the North American literature prior to Linsley & Chemsak (1972) concluding that the North American and Eurasian forms represented the same species refers to this species as R. lineatum.
Linsely, E. G. & J. A. Chemsak. 1972. Cerambycidae of North America, Part VI, No. 1. Taxonomy and classification of the subfamily Lepturinae. University of California Publications in Entomology 69:viii + 1–138, 2 plates.
Yanega, D. 1996. Field Guide to Northeastern Longhorned Beetles (Coleoptera: Cerambycidae). Illinois Natural History Survey Manual 6: x + 1–174 [preview].
Copyright © Ted C. MacRae 2013