Seasonal fluctuations of population density of the cabbage aphid, Brevicoryne brassicae (L.), in the Netherlands, and the role of its parasite, Aphidius (Diaeretiella) rapae (Curtis)

Brevicoryne brassicae (L.) had 4-14 generations in the year, it overwintered in the egg stage on cruciferous plants and reached a peak of abundance on brussels sprouts in July, rapidly declined and increased again in September-October. The mid-season decline proved to be due to increased mortality rather than to reduction of reproduction or to migration.

Abiotic factors and biotic factors, entomophagous fungi, predators and parasites, Aphidius rapae, (Curtis) were all involved.

A. rapae was an internal parasite, only one adult emerged from each host. The parasite overwintered as a last-instar-larva inside the host. The adults emerged in spring. There were 5-11 generations a year each lasting 14-16 days. Eggs were deposited singly and at random in infested aphids. Half-grown nymphs seemed to be preferred to other stages and wingless to winged.

Parasitism eliminated reproduction completely if it occurred early in the host development but not necessarily if it were delayed until the fourth instar or later. Host development was always delayed.

Parasitism caused only a small proportion of total aphid mortality though it sometimes reached 80-90 % of the stem-mothers on overwintered brussels sprouts. Hyperparasites, like Charips ancylocera, reduced effectiveness. Also the slower development of A. rapae than of B.brassicae reduced its effectiveness.