Natural Control of the Cereal Stemborers Busseola Fusca and Chilo Partellus in South Africa

Abstract

The stemborers Busseolafusca (Fuller) (Lepidoptera: Noctuidae) and Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae), are the most important pests of maize and grain sorghum in South Africa. Parasitoids, pathogens and predators curtailed populations of B. fusca and C. partellus on the crops but their activity was not enough to reduce pest populations to below-economic-damage levels. They also could not prevent the dispersal and subsequent widespread distribution of C. partellus after its introduction into South Africa. Nine parasitoids and hyperparasitoids have been recorded attacking both borer species. These parasitoids are probably more habitat-specific with regard to foraging for hosts than they are host-specific, as they appear to have moved from their indigenous hosts to use C. partellus as a new alternative host. Between 1977 and 1993 three braconids, one eulophid, three ichneumonids, two trichogrammatids and four tachinids were introduced into South Africa from 11 countries for biological control of stemborers. Recoveries of parasitoids were made only short periods after releases and in the vicinity of the release sites. Cold and dry winters, with night temperatures often falling to below freezing, prevail in the Highveld Region which is the major summer-grain-producing area of South Africa. These conditions, and the fact that borers enter diapause in the dry stalks for 7 months in winter, when the crops or other alternative host plants such as grasses do not grow, hamper the establishment of exotic natural enemies in the Highveld Region.