Frequency of alleles conferring resistance to a Bacillus thuringiensis toxin in a philippine population of Scirpophaga incertulas (Lepidoptera: Pyralidae)

Using the F₂ screen methodology, we estimated the frequency of alleles conferring resistance to the CrylAb toxin of Bacillus thuringiensis Berliner in a Philippine population of the stem borer Scirpophaga incertulas (Walker). Evaluation of >450 isofemale lines for survival of F₂ larvae on crylAb plants did not detect the presence of an allele conferring a high level of resistance. The frequency of such an allele in the sampled population was conservatively estimated to be <3.6 × 10^-3 with 95% confidence and a detection probability of 94%. However, there was evidence of the presence of alleles conferring partial resistance to CrylAb. The frequency of alleles for partial resistance was estimated as 4.8 × 10^-3 with a 95% CI between 1.3 × 10^-3 and 1.04 × 10^-2 and a detection probability of 94%. Our results suggest that the frequency of alleles conferring resistance to CrylAb in the population of S. incertulas sampled is not too high to preclude successful implementation of the high dose/refuge resistance management strategy.