LOCALIZED FAILURE OF A WEED BIOLOGICAL-CONTROL AGENT ATTRIBUTED TO INSECTICIDE DRIFT

In South Africa, a bud-feeding apionid weevil, Trichapion lativentre ( Beguin Billecocq), is one of three introduced insect herbivores that a re responsible for the successful biological control of Sesbania punic ea (Cav.) Benth., a leguminous weed of South American origin. With few exceptions, the annual seed crop of S. punicea has declined by more t han 98% since T. lativentre became established on the weed during the late 1970s. However, there are several localized infestations of S. pu nicea in the Olifants river valley where T. lativentre has been much l ess successful than elsewhere in the country, and where pod and seed p roduction by S. punicea is still prolific. In this valley, S. punicea often occurs in close proximity to citrus orchards, many of which are sprayed with organophosphate insecticides in spring and early summer ( August-October). In the vicinity of sprayed citrus orchards, populatio ns of T. lativentre remained low during the early summer (the main per iod of bud-burst for S. punicea) and weevil numbers only increased lat er, when insecticide treatments in the orchards had almost ceased. As a result, the incidence of damaged flower-buds was lowest, and pod pro duction was highest, on S. punicea plants in the vicinity of sprayed c itrus orchards. The negative effects of the insecticides, as measured by higher levels of pod production on the weed, decreased exponentiall y with increasing distance up to 300 m from the nearest sprayed orchar d.