Role of Parasitoids in Managing Diamondback Moth in the Cameron Highlands, Malaysia

Three major primary parasitoids of the diamondback moth, Plutella xylostella (L.), are found in the Cameron Highlands, Malaysia. Cotesia plutellae (Kurdjumov) was discovered in the early 1970s, Diadegma semiclausum (Hellén) and Diadromus collaris (Gravenhorst) were introduced in the mid 1970s from New Zealand and Australia. Field studies in the early 1980s showed that C. plutellae was the dominant parasitoid. This was contrary to laboratory studies which showed that D. semiclausum was an intrinsically superior parasitoid. In the Cameron Highlands, farmers sprayed insecticides frequently, often at concentrations far exceeding recommended dosages. In 1987, Singapore imposed restrictions on excessive levels of pesticide residues in crucifers. This together with high levels of insecticide resistance in the DBM resulted in farmers switching to the use of Bacillus thuringiensis, resulting in an unprecedented development for the management of diamondback moth. Even with reduced usage of chemical insecticides farmers were able to harvest good crops. Reduction in use of chemical insecticides allowed the primary parasitoids to realize their potential. The impact of biological control is manifested in: a) lower population of DBM despite less usage of insecticides; b) D. semiclausum became the dominant parasitoid; c) farmers realized that they need not be dependent on insecticides. Hence after more than 10 years, the impact of D. semiclausum, which was masked by excessive insecticide usage, was realized. Other parasitoids, predators and microbial agents probably act in consonance to suppress the DBM population. The experience in the Cameron Highlands emphasized the central role of parasitoids in managing diamondback moth, The strategy in managing this pest is to build up a core of effective parasitoids and supplement the action of parasitoids with use of B. thuringiensis when necessary. Introduction It was evident from reports of insecticide resistance in diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Yponomeutidae) (Henderson 1957; Lim 1972; Ooi and Sudderuddin 1978; Sudderudin and Kok 1978), that the approach adopted by cabbage farmers in the Cameron Highlands had to change. As the pest is an exotic insect, a likely approach was to import natural enemies of the DBM from its native home. This approach was attempted in New Zealand (Muggeridge 1930; Hardy 1938) and resulted in significant decline in DBM populations there (Todd 1959). Similarly, parasitoids of DBM were introduced into Australia and contributed to a useful level of control (Wilson 1960). Nearer home, an ichneumonid was introduced from New Zealand to Indonesia in 1950 and was an important parasitoid of DBM (Vos 1953; Sastrosiswojo and Sastrodihardjo 1986). These records suggested that biological control of DBM is feasible. With that in mind, a biological control program for DBM was initiated in 1975. This paper reviews the status of the biological control program for DBM in Malaysia with a view to establishing a strategy for the role of parasitoids in the overall management of DBM.