The Impact of Parasitoids on Plutella Xylostella Populations in South Africa and the Successful Biological Control of the Pest on the Island of St. Helena

Diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is the most injurious insect pest of brassica crops throughout the world. In many countries it has developed resistance to almost every synthetic insecticide used against it including Bt formulations. In addition to resistance, the destruction of its natural enemies through indiscriminate use of broadspectrum insecticides is considered responsible for its high pest status. Population studies of P. xylostella and its parasitoids in the Eastern Cape, Gauteng and North-West Provinces of South Africa revealed that the pest is naturally controlled if insecticides are not used. A total of 3 egg-larval parasitoids, 8 larval parasitoids, 4 larval-pupal parasitoids, 6 pupal parasitoids and 12 hyperparasitoids have been identified as being associated with P. xylostella in South Africa. An insecticide check method was used to assess the impact of parasitoids on levels of infestations by P. xylostella. In the sprayed plots parasitism of P. xylostella larvae and pupae fluctuated between 5-10% whereas in the untreated plots parasitism peaked above 90%. As a result population levels of P. xylostella on the sprayed plants were about five times higher than on the control plants, which is an indication that parasitoids played an important role in controlling the pest populations. Plutella xylostella was a severe pest on the Island of St Helena, South Atlantic Ocean. Farmers were heavily depended on chemical control, often spraying cocktails of several insecticides when the required control failed. A survey in brassica crops on St Helena revealed that natural enemies were not an important factor in controlling P. xylostella and that the only parasitoid on the Island was the larval-pupal parasitoid Diadegma mollipla (Holmgren) (Hymenoptera: Ichneumonidae). Following an agreement between NRInternational and the Plant Protection Research institute (PPRI) of South Africa two consignments of the larval parasitoid, Cotesia plutellae (Kurdjumov) (Hymenoptera: Braconidae), and the pupal parasitoid, Diadromus collaris Gravenhorst (Hymenoptera: Ichneumonidae), were shipped in 1999 from South Africa to St Helena. The parasitoids were mass reared on the Island and released on 10 different farms. An early survey of 19 farms (release and non-release sites) in 2000 indicated that both introduced parasitoids became established. Cotesia plutellae was found in 15 farms ________________________________ Successful Biological Control of Diamondback Moth in St. Helena Second International Symposium on Biological Control of Arthropods 133 with up to 80% parasitism and D. collaris on 5 farms with up to 55% parasitism. Further surveys during 2002 2004 indicated very low levels of P. xylostella populations. However, C. plutellae cocoons were present throughout the Island which is an indication that parasitoids had been the cause for the decline in the pest populations. Farmers in St Helena reported that P. xylostella infestations remain low and that no chemical control has been necessary since 2001. This is a strong indication for the success of the biological control of P. xylostella on St Helena.