Powder-post Beetles: Got Wood?

Jesuit beetle Bostrychopsis jesuita, from PaDIL.

Back when I was collecting insects in the Australian arid zone, one of the more easily recognisable animals that we would regularly come across was the jesuit beetle Bostrychopsis jesuita. I have no idea what gives them the name 'jesuit beetle', but these relatively large black beetles with their spiny pronotum with its two 'horns' coming off the front is not likely to be confused with much else. (Actually, now I think about it, could it be the horns that make them Jesuits? In which case, ouch.)

Bostrychopsis jesuita is one of the larger beetles belonging to the Bostrichidae, a family commonly going by the vernacular names of auger beetles or powder-post beetles. Both these vernacular names refer to the fact that many species in the family, particularly as larvae, are wood-borers (not fortune-tellers—that would make them augur beetles), with the ability to turn sapwood into a powdery frass. As a result, a number of species are notable timber pests. Other species, particularly the larger grain borer Prostephanus truncatus and the lesser grain borer Rhyzopertha dominica, are seed-feeders and pests in stored grain. A single genus, Endecatomus, is mycophagous, feeding on polypore fungi growing on dead hardwood trees (Liu & Schönitzer 2011). Bostrichids belong to a larger group of beetles, the Bostrichoidea, that are superbly adapted to feeding on dry foodstuffs (other members of the Bostrichoidea include the spider beetles of the Ptinidae and the carpet beetles of the Dermestidae). Modifications of the gut in most bostrichoids allow them to efficiently resorb water from the gut contents, and they can obtain pretty much all the moisture they need direct from the air.

Lesser grain borers Rhyzopertha dominica, from the US Department of Agriculture.

The bostrichids are divided between a number of subfamilies, members of which appear quite distinct from one another. Members of the Bostrichinae (to which Bostrychopsis belongs) and Dinoderinae are cylindrical and robust, with short sturdy legs. In some species, the rear of the body appears sharply cut off by a flat declivity in the rear of the elytra; this declivity can be by males to block the entrance to holes in the wood while the female is laying eggs deeper within (Lawrence & Britton 1991). Superficially, Bostrichinae can bear a close resemblance to another, unrelated group of wood-boring beetles, the bark beetles of the Scolytinae (I have to confess to confusing them myself when sorting specimens). The two groups can most readily be distinguished by their antennae, which are geniculate (elbowed) with a compact club in scolytines but non-geniculate with a loose clube in bostrichines.

A lyctine Trogoxylon impressum, copyright Udo Schmidt.

Members of other bostrichid subfamilies are not adapted to boring in wood as adults as well as as larvae, and as such are more elongate and less compact. The Lyctinae are more or less flattened, rectangular beetles that have been treated as a separate family in the past due to their distinct appearance. One group of lyctines, the Cephalotomini, are even more flattened than others, and are specialised for living as inquilines in the tunnels made by other bostrichids, feeding on their frass; their flattened bodies allow them to press themselves against the side of the tunnel and allow their host beetles to walk over them. Rather than boring deep into wood like bostrichines to lay eggs, females of species with non-boring adults use a long, flexible ovipositor to reach into cracks and other breaches in the tree's bark (Liu & Schönitzer 2011).

REFERENCES

Lawrence, J. F., & E. B. Britton. 1991. Coleoptera (beetles). In: CSIRO. The Insects of Australia 2^nd ed. vol. 2 pp. 543–683. Melbourne University Press.

Liu, L. Y., & K. Schönitzer. 2011. Phylogenetic analysis of the family Bostrichidae auct. at suprageneric levels (Coleoptera: Bostrichidae). Mitt. Münch. Ent. Ges. 101: 99–132.