Assessment of the biological control impact of seed predators on the invasive shrub Acacia nilotica (Prickly acacia) in Australia

An impact assessment study was undertaken to determine seed predation rates by two bruchid beetles, Bruchidus sahlbergi Schilsky and Caryedon serratus Olivier, on the invasive shrub Acacia nilotica (L.) Willd. ex Eel. The for mer bruchid was released as a biological control agent for A. nilotica, whe reas the latter is naturalized in Australia. We attempted to determine the dynamics and magnitude of bruchid predation, following a number of differin g reports of their effectiveness. To investigate the importance of seed ava ilability on bruchid numbers, we compared seed death in high- and low-seed- density habitats, both before and after pods had dropped from tree branches . Bruchid predation was initially low (<2%) in pods on tree branches in bot h habitats, but increased abruptly to 12% in riparian and 32% in nonriparia n exclosures by the third collection date. Bruchid numbers then crashed to below 2% in both habitats, when pods dropped to the ground. B. sahlbergi pr edation later increased markedly (up to 65%) again at high-pod-density site s. No further increase in predation took place beyond this level. Seed pred ation by C. serratus was found to be minimal (<5% of seeds infested) throug hout the monitoring period. We suggest that seed predation by the bruchids, prior to pod drop and removal by cattle, is insufficient to cause major im pacts on A. nilotica populations. Bruchid seed damage is likely to be signi ficant only in situations with low cattle numbers, where cattle cannot remo ve the majority of fallen seed pods or where cattle are excluded. Exclusion of cattle from some areas within A. nilotica-invaded landscapes to potenti ally increase bruchid effectiveness is proposed. The demographic impact of optimal A. nilotica seed losses is discussed. (C) 2001 Academic Press.