SEASONAL INCIDENCE OF APHIDS AND THE APHID PARASITOID DIAERETIELLA-RAPAE (MINTOSH) (HYMENOPTERA, APHIDIIDAE) ON RAPESEED IN TENNESSEE

Aphids and their parasitoids were sampled in seven and eight commercial rapeseed fields in 1990 and 1991, respectively. Whole-plant sampling was used for aphids during the budding plant stage. Sweep-net sampling was used for aphids and their parasitoids during the flowering and ripening plant stages. Three species of aphids, turnip aphid, Lipaphis erysimi (Kaltenbach); green peach aphid, Myzus persicae (Sulzer); and cabbage aphid, Brevicoryne brassicae (L.), were collected. The turnip aphid was the most abundant species collected in 1990 and 1991, 99.9 and 86.8%, respectively. Turnip aphid populations peaked (80%) most frequently during the flowering stage, and visual observations taken of feeding damage noted stunted growth, reduced pod formation, and uneven stand maturity. Turnip aphids were parasitized by the wasp parasitoid, Diaeretiella rapae (M'Intosh) (Hymenoptera: Aphidiidae). In 1990, the total D. rapae density was 883 wasps per 50 sweeps and this was 24 times higher than the total 1991 density of 37 wasps per 50 sweeps. Aphid infestations were observed to cause substantial damage to rapeseed stands in 1990. However, the lack of quantitative yield-loss data precludes the effective integration of natural and chemical control strategies on rapeseed.