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.