Oviposition patterns and egg mortality in Acanthoscelides macrophthalmus (Chrysomelidae: Bruchinae), a biological control agent of Leucaena leucocephala (Fabaceae) in South Africa

摘要

Deliberately introduced for agroforestry, the invasive tree Leucaena leucocephala (Lam.) de Wit (Fabaceae) is naturalized in several countries worldwide, including parts of South Africa. The seed beetle Acanthoscelid.es macrophthalmus (Schaeffer) (Chrysomelidae: Bruchinae) was inadvertently introduced to several countries via contaminated seed, but was released as a biological control agent in South Africa. Although widely established in South Africa, its efficacy may be curtailed because damage is reputed to be confined to canopy-held seeds, with dehisced seeds escaping damage on the ground. To clarify the beetle's potential, we investigated: (i) its selection of oviposition sites when presented with pods bearing seeds, loose seeds and empty pods in different locations (canopy- versus ground-held); and (ii) the effect of seed location on egg mortality. The beetles oviposited on pods bearing seeds, loose seeds and empty pods in both locations but showed significant preferences for pods bearing seeds, regardless of location. Egg mortality was high (66 % overall) and caused largely by predation, but did not differ between canopy- and ground-held seeds. Dehisced seeds are thus more likely to escape damage due to the beetle's greater association withpods bearing seeds, which occur mostly in the canopy. While not affected by seed location, egg predation could affect beetle population densities. Although oviposition patterns and egg predation may constrain the efficacy of A. macrophthalmus, seed damage in relation to seed availability still needs to be quantified in the field.