Biological control of diamondback moth in Asia

Diamondback moth, Plutella xylostella (L.), is considered as the most destructive pest of crucifers in Asia, especially tropical to subtropical parts of the continent. Unlike Europe, where it originated, there are practically no native, pest-specific natural enemies present in this part of Asia. This coupled with increasing year-round cultivation of crucifer vegetables and intensive use of insecticides - which further eliminates beneficial arthropods - has resulted in diamondback moth gradually becoming a serious limitation to crucifer cultivation. Although the pest was known to occur in the region from the beginning of the twentieth century, serious efforts in biological control began in the 1950s with the introduction of a larval parasitoid, Diadegma semiclausum, from Europe into Indonesia. Because of continued use of insecticides, the beneficial effects of this parasitoid were not realized until the mid 1970s, whereupon this ichneumonid along with the larval parasitoid, Cotesia plutellae, were introduced in the highland areas of Malaysia. Region-wide efforts began in the late 1980s when the Asian Vegetable Research and Development Center in Taiwan initiated collaborative biological control research and development activities with various countries ranging from Pakistan in the west to the Philippines in the east. Through this program, five hymenopterous parasitoids, D. semiclausum, C. plutellae, Oomyzus sokolowskii, Microplitis plutellae, and Diadromus collaris, imported from Europe, West Africa, United States or within Asia were introduced into problem-plagued vegetable-growing areas of various countries. Some of these natural enemies have been established in the cooler highlands (D. semiclausum and D. collaris) while others in hotter lowlands (C. plutellae, O. sokolowskii, and M. plutellae) resulting in a gradual reduction in pest damage and pesticide use, especially in the highlands. In the lowlands, farmers' use of chemical insecticides for the control of other crucifer pests such as Crocidolomia binotalis, Hellula undalis, and Phyllotreta striolata, results in mortality of parasitoids leading to resurgence in diamondback moth damage. Continued use of largely ineffective insecticides is the greatest challenge to the biological control of diamondback moth in the region. Although use of biopesticides will help to protect the natural enemies and give a level of control similar to that obtained currently by chemical pesticides, unavailability of such products, beyond relatively expensive ones based on Bacillus thuringiensis, makes this task rather difficult. (Résumé d'auteur