A review of recent efforts at biological control of Caesalpinia decapetala (Roth) Alston (Fabaceae) in South Africa

A seed-feeding beetle, Sulcobruchus subsuturalis (Pic) (Coleoptera: Chrysomelidae: Bruchinae), was released in South Africa in 1999 as a biological control agent against the invasive, leguminous, scrambling shrub Caesalpinia decapetala (Roth) Alston (Fabaceae). Despite being easy to rear in the laboratory and having been released in large numbers (>350 000) and widely distributed at many field sites, the beetle remains scarce where present and has failed to persist at many of the original release sites. Although eggs are oviposited readily on loose seeds in dishes, the few eggs that were found at release sites in the field were always on seeds still attached to pods hanging on plants. High levels of predation and parasitism were recorded in the field, which may be hindering population expansion by the beetle. Clarity about the taxonomic status of S. subsuturalis, and uncertainty as to the centre of origin of the weed, have led to suspicions that C. decapetala may exist as several biotypes, and that the local variety may not be a suitable host for the form of S. subsuturalis that has been imported into South Africa. These uncertainties should be addressed, together with additional efforts to determine what is happening to the agent in the field. Other phytophagous species from different feeding-guilds need to be investigated as potential biological control agents.