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

1. The stability of resistance to Bacillus thuringiensis Crystal (Cry) toxi ns in highly and moderately resistant sub-populations of a Plutella xyloste lla field population (SERD4) was compared under laboratory conditions. The relative rate of decrease in resistance was greater in a highly resistant C ry1Ac-selected population than in moderately resistant Cry1Ab- and Cry1Ca-s elected populations. 2. The intrinsic rate of population increase (rm) was similar in all popula tions tested. 3. These results suggest that there are no obvious overall fitness benefits as the frequency of the resistance alleles is reduced.