Utilization of an unpredictable food source by Melanterius ventralis, a seed-feeding biological control agent of Acacia longifolia in South Africa

A seed-feeding weevil, Melanterius ventralis (Coleoptera: Curculionidae), has been introduced into South Africa to supplement a gall wasp,Trichilogaster acaciaelongifoliae (Hymenoptera: Pteromalidae), in the biological control programme against an alien invasive tree, Acacia longifolia (Mimosaceae), from Australia. The gall wasp debilitates most of the flower buds on A. longifolia andreduces seed-set by >95%. The intended rolefor M. ventralis is to destroy theresidual seeds. To achieve this, the gravidfemales need to be able to locate a food sourcethat is both heterogeneously dispersed andfrequently scarce due to damage caused by T.acaciaelongifoliae. Observations showedthat M. ventralis females are meticulousin choosing sites to oviposit so that larvae donot become overcrowded and food limited. Cagetests and field observations revealed thatfemales located pods regardless of density andposition, and that the duration of time spenton branches was proportional to the number ofpods on the branches and to the condition ofpods. The females spent little time onbranches of acacia species other than A.longifolia, but some time was spent onbranches with pods of A. melanoxylon thathad been coated with juice extracted from A.longifolia. All indications are that M.ventralis has the attributes needed to bean excellent supplementary biological controlagent to T. acaciaelongifoliae and theprogress of the weevil continues to be monitored.