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 commercia l rapeseed fields in 1990 and 1991, respectively. Whole-plant sampling was used for aphids during the budding plant stage. Sweep-net samplin g was used for aphids and their parasitoids during the flowering and r ipening 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 a nd 86.8%, respectively. Turnip aphid populations peaked (80%) most fre quently during the flowering stage, and visual observations taken of f eeding 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 ti mes higher than the total 1991 density of 37 wasps per 50 sweeps. Aphi d infestations were observed to cause substantial damage to rapeseed s tands in 1990. However, the lack of quantitative yield-loss data precl udes the effective integration of natural and chemical control strateg ies on rapeseed.