Reassessment of the biology and host range of Acanthoscelides macrophthalmus (Chrysomelidae: Bruchinae), a seed-feeding beetle released for the biological control of Leucaena leucocephala in South Africa.
Introduced for agroforestry, Leucaena leucocephala (Lam.) de Wit (Fabaceae) has become naturalized in several countries worldwide and constitutes an 'emerging weed' in South Africa. The seed beetle Acanthoscelides macrophthalmus (Schaeffer) (Chrysomelidae: Bruchinae) was inadvertently introduced to many of these countries via contaminated seeds, but was deliberately introduced into South Africa for biological control. Following an isolated report of non-target feeding in the field in southeastern Asia, this study was aimed at re-evaluating aspects of the beetle's biology, in particular the likelihood of non-target attacks on native mimosoid Fabaceae that are closely related to L. leucocephala. A preliminary field assessment of the beetle's susceptibility to native parasitoids was also undertaken. The beetle's life cycle includes several attributes (e.g. rapid reproductive maturation, high rates of oviposition) that favour high population densities in the field. Although adults oviposited on the seeds of all test plant species during no-choice tests, oviposition was significantly higher on L. leucocephala. While several hatching larvae survived to adulthood on L. leucocephala, single adult individuals were reared from the seeds of each of three non-target species. Also, during multichoice tests, the beetles displayed strong oviposition preferences for L. leucocephala. Two hymenopteran parasitoid species were reared from seeds containing larvae which were exposed in the field, but overall levels of parasitism were low. These results confirm that the host range of A. macrophthalmus is largely restricted to the genus Leucaena, that native mimosoid Fabaceae are unlikely to be at risk and that the decision taken in 1999 to release the beetle was justified.