| Through a series of field and laboratory experiments, I explored ways of reducing reliance upon insecticide spraying while fostering control by natural enemies using a broccoli-aphid-natural enemy complex. Demographic statistics developed for the aphid parasitoid, Diaeretiella rapae (McIntosh) and two hosts, the cabbage aphid, Brevicoryne brassicae (L.), and the green peach aphid, Myzus persicae (Sulzer), indicated that M. persicae was a better host for D. rapae. Parasitoids reared on M. persicae had higher intrinsic (rm) and finite rates of increase and shorter generation and doubling times than when reared on B. brassicae.; Laboratory exposure of D. rapae adults to the selective insecticide pymetrozine at the recommended field rate (0.2128 a.i./ha) increased rm 11% while exposure to the adjuvant Sylgard 309 reduced rm 18%. When combined, the chemicals acted synergistically, reducing rm 39%. The mummy stage afforded some protection to the parasitoid as exposure to both chemicals resulted in a 13% reduction in rm.; D. rapae alone, Coccinella septempunctata L. alone, and a combination of both organisms (ratio of 1:50 predator/parasitoid:aphid) in the laboratory led to a 38, 58, and 66% reduction in the level of aphid infestation, respectively. Pymetrozine alone at 1/64 field rate (0.0038 a.i./ha) caused an 85% reduction and the highest reduction (98%) was obtained when the low dose of pymetrozine was combined with both natural enemies. In caged field experiments, C. septempunctata alone in a ratio of 1:50 aphids led to a 43% reduction in the level of aphid infestation. D. rapae alone and the two natural enemies together did not reduce the aphid infestation. Pymetrozine [½ field rate (0.106g a.i./ha)] led to a 99.8% reduction in aphid numbers and was too effective for the potential additive reduction of aphids by biological control agents to be evident.; Release of C. septempunctata at 1:40, 1:20 and 1:10 C. septempunctata:aphid ratios in the field led to a 49, 79, and 89% reduction in the level of aphid infestation, respectively. Pymetrozine alone at 1/4 field rate (0.053g a.i./ha) was still effective and led to a 96% reduction in aphid infestation. |
