A winter longhorned beetle

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 | Reynolds Co., Missouri

Rhagium inquisitor overwintering adult | Reynolds Co., Missouri

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.

Adults overwinter in cells lined with frass and fibrous wood shavings.

Adults overwinter in cells lined with frass and fibrous wood shavings.

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

11 thoughts on “A winter longhorned beetle

  1. here is on Pinus too, specially at low altitude.
    collected few adults and found around 100 pupae 2-3 week ago, on Abies alba.
    Strange on the some log of Abies found one specimen of Rhagium mordax (in pupal cell), here this one is another species to be found in winter but until now always found this one on/under Fagus bark.

        • I presume you mean that it was introduced by humans, which is an interesting thought. If true, it would have to be among the very earliest such introductions, as the type locality of lineatus (Olivier 1795) is given as “America” and Linnaeus’ type from Europe is even earlier. There are actually a number of North American cerambycids with Holarctic distribution, including not only R. inquisitor but also Tragosoma depsarium (Linnaeus), Asemum striatum (Linnaeus), Arhopalus rusticus (Linnaeus), Acmaeops pratensis (Laicharting), Pachyta lamed (Linnaeus), Judolia sexmaculata (Linnaeus), and Saperda populnea (Linnaeus). The last is associated with with Populus, but all of the rest are associated with conifers. I’m not aware of any literature suggesting R. inquisitor or any of these other species are, in fact, introduced to North America, and R. inquisitor in particular seems not very prone to this because of its strictly under-the-bark habit.

  2. North American Rhagium inquisitor may be a species-in-making, since there are ecological differences in which tree they prefer. It would be useful to look at their genetics to see if they are consistent with this hypothesis.

    • I agree molecular studies would be interesting to conduct. I expect some divergence since the populations have been separated for some time – the question is how long they have been separated and whether genetic divergence has resulted in morphological/ecological distinctiveness. Right now there seems to be little or no morphological distinctiveness, but the ecological separation you mention might be approaching significance.

      • I agree with Gunnar. Any new introduced species will be a species-in-making, if they find new host plants in their new environment.


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