Melanterius Weevils

Here in the Antipodes, we have a long history of environmental upheaval from exotic taxa unwisely released. As a result, one can't help but feel an odd twinge of perverse patriotism when hearing of the inverse, some native of the Antipodes causing grief elsewhere. In South Africa, Australian acacias have become something of an issue, inciting a search for potential control agents. Among the candidates selected are weevils of the genus Melanterius.

Melanterius servulus, copyright Sally Adam.

Melanterius is a diverse genus of small black or brown weevils (ranging from about three to seven millimetres in length) that feed as both adults and larvae on the developing seeds of acacias. About eighty species have been recognised in the genus to date and possibly many more remain to be described. In general, Melanterius weevils are heavily punctate, usually without prominent hairs but with a covering of scales. The rostrum is reasonably long, reaching more or less back to the mesosternum at rest but not sitting in a distinct ventral groove, and may be variably curved (going by figures in Zimmerman 1992).

Melanterius semiporcatus, copyright Victor W. Fazio III.

As with other weevils, the prominent rostrum is used by females to chew into an appropriate spot on the host plant, in this case chewing holes into the developing acacia seed pods, into which eggs are laid. Melanterius species go through one generation per year. Larvae burrow into and feed on the developing seeds before emerging and dropping to the ground to pupate in the soil. Mature adults emerge well before the host acacias begin to set seeds, usually having to wait about six months (Auld 1989). They usually spend the intervening period largely inactive, sheltering in concealed places close to the host plant and occasionally emerging to briefly feed on developing buds.

Under peak conditions, Melanterius infestations may cause a complete failure of seed production. No wonder, then, that they have been considered a worthwhile instrument of biological control.

REFERENCES

Auld, T. D. 1989. Larval survival in the soil and adult emergence in Melanterius Erichson and Plaesiorhinus Blackburn (Coleoptera: Curculionidae) following seed feeding on Acacia and Bossiaea (Fabaceae). Journal of the Australian Entomological Society 28: 235–238.

Zimmerman, E. C. 1992. Australian Weevils (Coleoptera: Curculionoidea) vol. 6. Colour plates 305–632. CSIRO Australia.

Opening Dors

My current dayjob mostly revolves around identifying and counting dung beetles. When Europeans settled Australia, they brought their farm animals with them. Unfortunately, the large piles of dung produced by cattle and horses proved rather daunting to native scavengers used to the more compact droppings of kangaroos and possums. And if you've ever experienced an Australian summer, you'll know that flies are definitely a thing. To help with this situation, Australia has had a long-running programme introducing exotic dung beetles that are better able to clean up after livestock. Most of these are members of the typical dung beetle family Scarabaeidae but one species, Geotrupes spiniger, represents a different subgroup of the superfamily Scarabaeoidea. These are the earth-boring dung beetles or dor beetles of the Geotrupidae.

Dor beetle Geotrupes spiniger, copyright Udo Schmidt.
The geotrupids are medium-sized to very large beetles, ranging in size from half a centimetre to 4.5 cm in length (Jameson 2002). Like many other members of the Scarabaeoidea, they have broad fore legs used for digging. Their short, eleven-segmented antennae end in the asymmetrical club typical of scarabaeoids but they may be distinguished from other families in that the basal segment of the three-segmented club is expanded to form a 'cup' against which the other segments may be tightly closed. The body of geotrupids is strongly convex, and is smooth and shiny dorsally but hairy underneath. In many species, the males may bear elaborate horns and/or processes on the head and pronotum.

Male Taurocerastes patagonicus, copyright Nicolás Lavandero.

Despite their size, geotrupids are secretive animals, spending most of their time in burrows underground (which may be up to three metres in depth) and usually only emerging at night. Various species feed on animal dung or decaying matter; some feed on subterranean fungi. In at least some species, eggs are laid in brood chambers within the parent's home burrow and multiple life stages may share a single burrow. Burrows may also be shared between multiple adults when conditions demand. Though adults do not directly tend to larvae, they may stock brood chambers with food supplies. In some Australian species of the subfamily Bolboceratinae, females lay a single gigantic egg at a time that may be up to 56% the size of its layer (Houston 2011). Larvae hatching from such an egg are able to develop right through to maturity without feeding.

Adult geotrupids produce a stridulating noise when disturbed which is the origin of the alternate vernacular name of "dor beetle" ("dor" being an old word for a buzzing insect). Larvae may or may not be capable of stridulation, depending on the species.

Male Blackburnium rhinoceros, copyright Edward Bell.

The classification of geotrupids is the subject of ongoing investigation. A recent classification divides the family between three subfamilies, the widespread Geotrupinae and Bolboceratinae and the South American Taurocerastinae. Morphological differences between these subfamilies, particularly at the larval stage, have lead some researchers to question whether the Geotrupidae in the broad sense represents a monophyletic group. Molecular analyses thus far seem ambiguous; an analysis by McKenna et al. (2015) placed geotrupids as part of a polytomy near the base of the scarabaeoids. As an aside, my supervisor recently asked myself and a retired colleague whether Geotrupes spiniger was the only species of geotrupid found in Australia. I replied "yes", our colleague responded "no". Our conflict, of course, was based on whether Australia's wide diversity of Bolboceratinae contributed to the count.

REFERENCES

Houston, T. F. 2011. Egg gigantism in some Australian earth-borer beetles (Coleoptera: Geotrupidae: Bolboceratinae) and its apparent association with reduction or elimination of larval feeding. Australian Journal of Entomology 50: 164–173.

Jameson, M. L. 2002. Geotrupidae Latreille 1802. In: Arnett, R. H., Jr, M. C. Thomas, P. E. Skelley & J. H. Frank (eds) American Beetles vol. 2. Polyphaga: Scarabaeoidea through Curculionoidea pp. 23–27. CRC Press.

McKenna, D. D., B. D. Farrell, M. S. Caterino, C. W. Farnum, D. C. Hawks, D. R. Maddison, A. E. Seago, A. E. Z. Short, A. F. Newton & M. K. Thayer. 2015. Phylogeny and evolution of Staphyliniformia and Scarabaeiformia: forest litter as a stepping stone for diversification of nonphytophagous beetles. Systematic Entomology 40: 35–60.