A COMPARISON OF ATTACK RATES IN A NATIVE AND AN INTRODUCED POPULATIONOF THE PARASITOID COTESIA-GLOMERATA

The attack rate of a population of the braconid parasitoid Cotesia glo merata, introduced into the USA over 100 years ago as a parasitoid of Pieris rapae, was compared with that of a native British population, w hich normally attacks P. brassicae, and with that of a p. rapae specia list Cotesia rubecula. British C. glomerata attacked P. brassicae at a much higher rate than it attacked P. rapae. In comparison with Britis h C. glomerata, C. rubecula showed a higher attack rate with P. rapae. American C. glomerata attacked P. rapae at a slightly higher rate tha n 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 a nd American C. glomerata attacking P. rapae are discussed. They may be due to genetic or environmental effects. Egg load did nor appear to b e a factor limiting the number of hosts parasitized under the conditio ns of the experiments.