none Pieris brassicae; Pieris rapae; HYMENOPTERAN-PARASITES; Cotesia glomerata & C. rubecula; Attack rates compared in laboratory Cotesia glomerata; Cotesia rubecula; LEPIDOPTERAN-HOSTS; Pieris brassicae & P. rapae; Attack rates compared in laboratory The attack rate of a population of the braconid parasitoid Cotesia glomerata, introduced into the USA over 100 years ago as a parasitoid of Pieris rapae, was compared with that of a native British population, which normally attacks P. brassicae, and with that of a P. rapae specialist, Cotesia rubecula. British C. glomerata attacked P. brassicae at a much higher rate than it attacked P. rapae. In comparison with British C. glomerata. C. rubecula showed a higher attack rate with P. rapae. American C. glomerata attacked P. rapae at a slightly higher rate than did British C. glomerata, but not at as high a rate as that achieved by C. rubecula. The differences in each comparison were statistically significant. The possible causes of the differences between British and American C. glomerata attacking P. rapae are discussed. They may be due to genetic or environmental effects. Egg load did not appear to be a factor limiting the number of hosts parasitized under the conditions of the experiments.