Does height of exposure in the canopy influence egg mortality in Acanthoscelides macrophthalmus (Coleoptera: Chrysomelidae), a biological control agent of Leucaena leucocephala in South Africa?
Introduced into several countries worldwide for agroforestry, Leucaena leucocephala (Lam.) de Wit is a typical 'conflict species' that invades several riparian, forested and disturbed habitats in South Africa. The seed beetle Acanthoscelides macrophthalmus (Schaeffer) (Chrysomelidae: Bruchinae) was subsequently deployed as a seed-reducing agent to offset its excessive seed output. However, exploratory egg-exposure trials, in which native Acacia s.l. trees were used as 'surrogate host plants', suggested that high egg mortality is reducing the beetle's impact. The aims of this study were to (1) verify this suspicion by comparing the mortality of eggs exposed in L. leucocephala and Acacia nilotica (L.) Willd ex Del. stands and (2) determine whether height of exposure in the canopy (i.e. oviposition site selection) influences the different mortality factors. Although lower than in the exploratory trials, egg mortality was considerable, ranging from 30 to 50% in most instances. While unaffected by height of exposure, the overall egg mortality (i.e. all factors) was significantly lower in seeds exposed on L. leucocephala. Mortality was not only mostly attributed to parasitism (51% of mortality), but also predation (30%) and unknown factors (19%). While unexplained mortality and predation were not influenced by tree species or height of exposure, parasitism increased significantly with height above ground and was significantly lower in seeds exposed on L. leucocephala. Egg mortality is thus one of several factors that appears to be limiting the efficacy of A. macrophthalmus in South Africa.