First Resistance to Bt Documented

Diamondback moth in field populations developed resistance to Bacillus thuringiensis kurstaki (Bt) after repeated commercial foliar applications, according to researchers at the University of Hawaii. Exposed strains of the moth, Lepidoptera plutellidae, were found to have 20 to 40 times the resistance of unexposed strains. Bt has been used as a biological insecticide for 20 years with no previous resistance reported in the field. Today, Bt is widely used as one of the most promising alternatives to chemical insecticides.

Researchers conducted a long-term study of insecticide resistance in the diamondback moth, Plutella xylostella, to determine if populations in Hawaii vary significantly in their susceptibility to Bt. The survey began in 1986 and tested six moth populations that had previously received varying treatments of Bt. The most heavily treated SO population had been exposed to Bt 50 to 100 times starting in 1978 and ending in 1982. The other five populations (WO,PO,KH,LH,KM) received fewer than 10 treatments before the first study sample was taken. Also tested in the study were two laboratory colonies (LAB-P, LAB-L) that had been kept free of insecticide exposure. All populations were tested again two years later to determine if Bt resistance had increased. Moths from each population were exposed to Dipel (25.6 mg/L) to monitor their relative susceptibility.

Tests in 1986-87 showed that the frequently treated or "so" population was significantly more resistant to Bt than either of the two laboratory strains (Table 1). Application of Dipel killed 90 to 100 percent of the susceptible LAB-P and LAB-L colonies, 60 to 90 percent of the minimally treated field populations, and 60 percent of the heavily treated SO population. In 1989, only 35 percent of the SO larvae were killed. The untreated and minimally treated field populations exhibited no significant increase in resistance in 1989. Results show that the levels of resistance attained by the SO field populations may be high enough to substantially reduce field effectiveness.

The resistance problem comes at a time when Bt's popularity and usage are increasing. For example, recent advances in genetic engineering have included the in-sertion and expression of Bt toxin genes into major crops like cotton, tobacco, potato and tomato. The development of new Bt strains and altered host ranges may further increase its usefulness.

The key to managing Bt resistance is prevention through judicious application. Unless the principles of IPM and pesticide resistance management are used-methods such as tissue specific and conditional expression of toxin genes in genetically engineered crop cultivars, and incorporating untreated plant matter-the reliability of Bt may be undermined.

Tabashnik, B.E., N.L. Cushing, N. Finson, M.W. Johnson, 1990. Field development of resistance to Bacillus thuringiensis in diamondback moth (Lepidoptera: Plutellidae). Journal of Economic Entomology. 83: 1671-1676.

Tabashnik, B.E., N. Finson, M.W. Johnson, 1991. Managing Resistance to Bacillus thuringiensis. Lessons from the Diamondback Moth (Lepidoptera: Plutellidae).J. Econ. Entomol. 84(2): 1991.