Diamondback Moth Resistance to Bacillus thuringiensis in Hawaii

Resistance to the microbial insecticide Bacillus thuringiensis Berliner has evolved in field populations of diamondback moth, Plutella xylostella (L.), in Hawaii. Three field populations that had been treated repeatedly with commercial formulations of the sporecrystal protein complex of B. thuringiensis subsp. kurstaki developed significant resistance to it. The LC50 of the most resistant population was 25-36 times greater than the LC50s of susceptible laboratory strains and 4 1 times greater than the Lc50 of the most susceptible field population. Laboratory selection of three strains established from the most resistant field population caused 15-30-fold increases in LC50 in nine generations, resulting in 430-820-fold resistance compared with a susceptible laboratory strain. Resistance declined slowly in the absence of treatments. Development of resistance to a commercial formulation containing a mixture of B. thuringiensis toxins by field populations of DBM casts doubt on the ability of mixtures to retard evolution of resistance. Slow restoration of susceptibility in the absence of treatments suggests that rotations may not be especially effective for retarding resistance development. We urge judicious use of B. thuringiensis to conserve i ts