Aphidius colemani; Aphidius similis; SAMPLING-; PATHOLOGICAL-TECHNIQUES; Sampling population on hemipteran hosts, trap plants use; HEMIPTERAN-HOSTS; Rhopalosiphum padi; NEW-SOUTH-WALES; South; Sampling parasite population using trap plants Rhopalosiphum padi; PATHOLOGICAL-TECHNIQUES; Sampling hymenopteran parasite population, trap plants use; HYMENOPTERAN-PARASITES; Aphidius colemani & A. similis; Sampling parasite population using trap plants; NEW-SOUTH-WALES; South; Sampling hymenopteran parasite population using trap plants The activity of cereal aphid parasitoids in the southern half of the New South Wales wheat belt was assessed by exposing pottcd barley seedlings infested with Rhopalosiphum padi at selected sites for about 12 days in autumn and again in spring, 1991. Two parasitoid species, Aphiditis colemani and Aphidius similis were recovered at three out of2O sites in autumn (May),0 and at 15 of l9 sites in spring (September). The level of parasitism varied considerably and exceeded 4O% in individual pots at five sites in spring. none