Insecticidal control of Melanterius servulus (Coleoptera: Curculionidae), a potential biocontrol agent of Paraserianthes lophantha (Leguminosae), in commercial seed orchards of black wattle, Acacia mearnsii(Leguminosae)

A seed-feeding weevil, Melanterius servulus Pascoe, is considered suitable for release against the weed Paraserianthes lophantha. However, host specificity tests have indicated that M. servulus may also develop on the seeds of Acacia mearnsii (black wattle), which is both a weed and a crop plant in South Africa. Seed orchards, which supply the black wattle industry, may therefore need to be protected. The aim of this study was to determine whether the two pyrethroid insecticides presently used in wattle plantations against the wattle bagworm (Kotochalia junodi Heylaerts (Lepidoptera: Psychidae)) are effective against M. servulus. Since M. servulus has not yet been released in South Africa, field trials were not possible, and evidence that black wattle seed orchards can be protected from M. servulus was obtained indirectly, in two stages. Firstly, field trials showed that the two insecticides effectively protect the weed Acacia longifolia from a closely related weevil, Melanterius ventralis Lea, which has already been established as a biocontrol agent. Secondly, quarantine laboratory trials demonstrated that M. ventralis and M. servulus showed similar avoidance responses and similar levels of mortality when exposed to the insecticides. All the evidence indicates that the chemical protection of A. mearnsii seed orchards from M. servulus is possible.