Can reduced rates of pymetrozine and natural enemies control the cabbage aphid, Brevicoryne brassicae (Homoptera: Aphididae), on broccoli?

We investigated the potential of using reduced rates of the selective aphicide (pymetrozine) and two biological control agents both in the laboratory and in the field to control the cabbage aphid, Brevicoryne brassicae (L.). The natural enemies were an aphid parasitoid, Diaeretiella rapae ( M'Intosh) and the seven-spotted lady beetle, Coccinella septempunctata (L.). Biological control agents were used in a ratio of 1:50 predator/parasitoid: aphid. The use of D. rapae and C. septempunctata alone in the laboratory resulted in a 38 and 58% reduction in the level of aphid infestation, respectively, while the combination of both biological control agents led to a 66% reduction. Pymetrozine alone at 1/64 field rate (0.003 g a.i./ha) caused an 85% reduction of aphids in the laboratory, and when pymetrozine was combined with C. septempunctata a 90% reduction in the level of infestation was achieved. When pymetrozine was combined with D. rapae in the laboratory, an 84% reduction of aphids was achieved. The highest reduction of aphids (98%) was obtained when the low dose of pymetrozine was combined with both biological control agents in the laboratory. In caged field experiments, the use of C. septempunctata alone led to a 43% reduction in the level of infestation. Cages with D. rapae alone and both biological control agents did not reduce the level of infestation in comparison to the control. When pymetrozine was used at 1/2 field rate (0.106 g a. i./ha) a 99.8% reduction in aphid numbers was obtained. This rate of suppression with pymetrozine was too effective for the potential additive reduction of aphids by biological control agents to be evident. In order to tease apart the effect of biological control agents in aphid population control, a lower rate of pymetrozine should be employed.