INDIANMEAL MOTH (LEPIDOPTERA, PYRALIDAE) RESISTANCE TO DIFFERENT STRAINS AND MIXTURES OF BACILLUS-THURINGIENSIS

The potential for Indianmeal moths, Plodia interpunctella (Hubner), to evolve resistance to isolates of Bacillus thuringiensis other than subsp. kurstaki HD-1 was determined through laboratory selection experiments. Resistance developed quickly toward isolates HD-1 12 and HD-133 of subsp. aizawai and HD-198 of subsp. entomocidus, reaching levels of 28, 61, and 21, respectively, after almost-equal-to 20 generations of selection. However, the rates of progression were somewhat slower toward these isolates than toward HD-1, probably because of the different toxin composition of these other isolates. Simultaneous selection for resistance to a mixture of HD-1 and HD-133 resulted in resistance to both isolates, but the rate toward the mixture was somewhat slower than that to the individual isolates. An insect population that was already resistant to isolate HD-1 readily developed resistance to isolate HD-133 as well. The results demonstrate that mixtures of strains do not preclude the possibility of resistance and that their value in slowing resistance development may not be large.