Localized failure of a weed biological control agent attributed to insecticide drift
1995
Hoffmann, J.H. | Moran, V.C.
In South Africa, a bud-feeding apionid weevil, Trichapion lativentre (Beguin Billecocq), is one of three introduced insect herbivores that are responsible for the successful biological control of Sesbania punicea (Cav.) Beath., a leguminous weed of South American origin. With few exceptions, the annual seed crop of S. punicea has declined by more than 98% since T. lativentre became established on the weed during the late 1970s. However, there are several localized infestations of S. punicea in the Olifants river valley where T. lativentre has been much less successful than elsewhere in the country, and where pod and seed production 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, populations of T. lativentre remained low during the early summer (the main period of bud-burst for S. punicea) and weevil numbers only increased later, when insecticide treatments in the orchards had almost ceased. As a result, the incidence of damaged flower-buds was lowest, and pod production was highest, on S. punicea plants in the vicinity of sprayed citrus orchards. The negative effects of the insecticides, as measured by higher levels of pod production on the weed, decreased exponentially with increasing distance up to 300 m from the nearest sprayed orchard.
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