Biological control of Leucaena leucocephala (Lam.) de Wit (Fabaceae) in South Africa: a tale of opportunism, seed feeders and unanswered questions.
Leucaena leucocephala (Lam.) de Wit (Fabaceae) is a typical 'conflict species', possessing both useful and damaging attributes, that has become naturalized in several countries worldwide, following deliberate introductions for agroforestry. Considered one of the 'World's 100 Worst Invaders', the plant in South Africa is an incipient or 'emerging weed', with infestations mostly occurring in the eastern subtropical regions of the country. Deliberate biological control efforts have been confined to South Africa, although these have been opportunistic, unfunded and, given the plant's propensity for prolific seed production, have focused exclusively on the seed beetle, Acanthoscelides macrophthalmus (Schaeffer) (Chrysomelidae: Bruchinae). Following confirmation of host specificity, and after several low-intensity releases, the beetle has become widely established in KwaZulu-Natal, where releases were mostly made, but also in Gauteng, Mpumalanga and the North West provinces. A sap-sucking psyllid, Heteropsylla cubana Crawford (Psyllidae), was inadvertently introduced into South Africa, but has not had any impact on weed infestations. The biological control programme against L. leucocephala in South Africa is reviewed, including: (i) aspects of the target plant that are pertinent to its invasiveness and management; (ii) details pertaining to the two natural enemies that have featured so far; (iii) progress that has been achieved; and (iv) the direction of future research efforts.