THE STATUS OF DIAMONDBACK MOTH, PLUTELLA-XYLOSTELLA (LINNAEUS) (LEPIDOPTERA, PLUTELLIDAE), AND ITS PARASITOIDS ON CABBAGES IN SOUTH-AFRICA

Diamondback moth, Plutella xylostella (Linnaeus) (Plutellidae), was la st studied in South Africa in the Pretoria North district in 1947 by U llyett. In the present study the incidence and seasonal history of P. xylostella were studied in the same area. Populations were low (maximu m 0.4 larvae/plant) throughout the study period (February 1998 to Febr uary 1992) and declined during winter. Only one parasitoid, Apanteles halfordi Ullyett (Hymenoptera: Braconidae), was present throughout the study, in contrast to a complex of 14 insect parasitoid species recor ded by Ullyett. The latter complex included a Diadegma species (Hymeno ptera: Ichneumonidae) and A. halfordi which Ullyett regarded as the mo st and secondmost important species controlling P. xylostella. The pre sent study revealed that P. xylostella did not cause economic losses i n this study area, and was apparently controlled by a combination of c hemical measures and A. halfordi which caused high mortality (up to 90 %) at the low host-population levels. At a second study site at Dinok ana in the western Transvaal, however, epidemic outbreaks of P. xylost ella occurred from October to December 1991. The average numbers of la rvae/plant for middle-aged and mature cabbages were 36 and five respec tively, and caused total crop loss. At this study site, regular and ex cessive chemical applications had been made during the previous two ye ars (1990 and 1991) and recently transplanted two-week-old cabbages av eraged 39 eggs, 24 mines and 29 larvae per plant despite regular chemi cal applications. Plutella xylostella thus appears to have developed r esistance to pesticides in this region of South Africa. Features of P. xylostella and A. halfordi that make them an ideal biocontrol associa tion are examined and discussed.