Resistant cultivars are another option. Trials include imported Lepidoptera-resistant cauliflower and cabbage lines developed in the United States. But pesticide resistance alone does not make a plant commercially viable; Southeast Asian consumers dislike the thick, waxy, dark green leaves. At AVRDC, scientists had greater success with microbial and biological control. In 1973, a granulosis virus (GV) was isolated from a diseased DBM larva, and used to replicate the virus, which was then sprayed in suspension over live larvae. Unfortunately, heat and sunlight both deactivate the virus. India ink added to the suspension protected the virus from sunlight. But to beat the heat-sensitivity problem crucifers should be grown in the cold season.
Bacillus thuringiensis is a potent bio-control agent for DBM. The most potent strain, SAN 45, is commercially available.
D. eucerophaga was also applied to cabbage in 1985-86 on farms in three locations in Taiwan: Luchu (lowlands), Yangminshan and Wuling (highlands). All the farms were infested with insecticide-resistant DBM. During the trial farmers used no chemical pesticides, but did spray Bt. In Luchu, 35,166 cocoons of D. eucerophaga were introduced at three sites. Farmers stopped spraying pesticides and a native parasite, Apanteles plutellae, began to attack DBM along with the introduced parasite, D. eucerophaga.
Subsequently, AVRDC researchers collected and reared the native parasite in the laboratory and released 12,506 pupae in the three sites along with the D. eucerophaga. The combined parasitism was 27%. Yields did not increase, but farmers in the Luchu trials saved money by not using pesticides. In Yangminchan, 45,000 cocoons of both parasites were introduced. After one month, parasitism reached 75%. Again yields were not affected, though savings increased almost 30% over the previous year. In 1984, local farmers spent NT$47,000 (US$1,300) on pesticides. In 1986 expenditures on Bt decreased to NT$17,000 (US$470). In Yangminshan, researchers released 35,600 cocoons of D. eucerophaga and 97,200 cocoons of A. plutellae. Parasitism rates reached an 80% peak one week before harvest. A. plutellae was found to be more virulent than D. eucerophaga.
As a result of the trials, D. eucerophaga established itself in Wuling. A 1988 survey revealed that in Wuling D. eucerophaga parasitized 45.6% of the DBM population. A. plutellae parasitized 11.3%. Farmers in the region report using fewer pesticides and the DBM population has fallen. Of all the IPM efforts of AVRDC, this technique has the most potential for success.
Just the same, IIHR recommends spot spraying cabbage with 0.05% cartap hydrochloride to control DBM in the winter and spraying mustard with 0.1% dichloros to control the podborer (Helicoverpa armigera) and aphids. Scientists chose these pesticides because they both have low residual toxicity and fumigant properties. In the meantime, intercropping crucifers with non-crucifers, using no pesticides, requires more research and development.
K. Srinivasan and P. N. Krishna Moorthy. 1991. Indian Mustard as a Trap Crop for management of Major Lepidopterous Pests on Cabbage. Tropical Pest Management. Vol. 37, No, 1.
P.O. Box 205