Fitness costs and stability of resistance to Bacillus thuringiensis in a field population of the diamondback moth Plutella xylostella L.

Summary 1. The stability of resistance to Bacillus thuringiensis Crystal (Cry) toxins in highly and moderately resistant sub‐populations of a Plutella xylostella field population (SERD4) was compared under laboratory conditions. The relative rate of decrease in resistance was greater in a highly resistant Cry1Ac‐selected population than in moderately resistant Cry1Ab‐ and Cry1Ca‐selected populations. 2. The intrinsic rate of population increase ( r m ) was similar in all populations tested. 3. These results suggest that there are no obvious overall fitness benefits as the frequency of the resistance alleles is reduced.