POPULATION DYNAMICS OF THE CABBAGE APHID, BREVICORYNE BRASSICAE (HOMOPTERA: APHIDIDAE) AT VANCOUVER, BRITISH COLUMBIA: I. SAMPLING METHODS AND POPULATION TRENDS

Abstract

AbstractMaris Kestrel kale, Brassica oleracea L., was planted and sampled in two locations during 1977–78. The aphids Brevicoryne brassicae (L.) and Myzus persicae (Sulzer), syrphids in the genera Syrphus and Scaeva, the cecidomyiid Aphidoletes aphidimyza (Rond.), the parasite Diaeretiella rapae (M'Intosh), and the fungus Entomophthora aphidis Hoffman were observed in the samples. A volumetric technique for assessing aphid numbers was developed and evaluated. Peak numbers of B. brassicae differed by an order of magnitude between plots, but the pattern of monotonic increase in the spring, followed by an abrupt decline in the rate of increase, a mid-summer peak, and an autumn population decline was consistent throughout. The first major decline in the rate of increase of B. brassicae was correlated with the appearance of A. aphidimyza and syrphid larvae. Declining adult aphid weight, which suggested declining fecundity, increased alate production at high aphid densities, parasite pressure, and leaf fall added to the damping effect of predators. In autumn, cool temperatures and production of sexuals further reduced the rate of increase of B. brassicae.