Classical biological control of diamondback moth: the Malaysian experience

The diamondback moth (DBM), Plutella xylostella (L), has been recorded in Malaysia since 1925. From 1941, it has been the major pest of crucifers grown in the Cameron Highlands and in the lowlands. The insect occurs in high numbers in the drier periods of the year and control has been mainly with the use of insecticides. The indiscriminate use of insecticides on DBM has led to the development of resistance to almost all classes of pesticides including organophosphates, carbamate, pyrethroids, acylureas and Bacillus thuringiensis (especially subsp. kurstaki). In 1975, a biological control program for DBM was initiated. In the same year, a braconid parasitoid Cotesia plutellae (Kurdj.) was recovered in Cameron Highlands. Parasitism levels ranged from 12 to 36% which was not effective in managing populations of DBM. Between 1975 and 1978, four species of parasitoids were introduced into the Cameron Highlands from India, New Zealand, Australia and Indonesia. Of these, only two became established, namely, Diadegma semiclausum Hellen and Diadromus collaris Gravenhorst. Within two years, both these parasitoids were recovered in crucifer-cultivated areas in the Cameron Highlands, even though parasitization levels were low. For almost a decade after their release the importance of the parasitoids was not realized and many considered the role of the parasitoids as not important in the management of DBM. In 1987 and 1988, serious outbreaks of DBM occurred in the highlands and lowland areas and which led to the extensive use of pesticides. This resulted in high pesticide residue in leafy crucifers and rejection of most harvested crops by the consumers. Between 1985 and 1989, studies were conducted to develop an Integrated Pest Management (IPM) package for DBM both for the highlands and lowlands areas of Malaysia. The package using DBM larval threshold levels and level of parasitization as decision tools was succesfully introduced to the crucifer growers in 1989. Also, biological pesticides i.e. Bacillus thuringiensis were recommended when populations exceeded the economic threshold levels. Between 1989 to 1993, DBM numbers fluctuated between 0.5 to 1 larva per plant with parasitization levels of 30 to 60% in the crucifer growing areas in Cameron Highlands. The farmers also applied insecticides mainly, Bacillus thuringiensis at only 1 to 2 applications per crop cycle compared to 7 to 10 applications before the introduction of IPM package. Several factors contributed to the success of the biological control programme to manage DBM in Malaysia. These factors include (1) Establishment of mass rearing facilities for parasitoids of DBM and periodic field releases (2) Training programmes intitiated for extension workers and farmers to recognise natural enemies and understand biological control concepts (3) Constant monitoring of insecticide resistance in problem areas where outbreaks occurred (4) Training on pesticide application technology for growers, and (6) Commitment of the government to reduce pesticide usage and use biological control as the core approach in the IPM programme for vegetables in Malaysia.