Population studies ofBrevicoryne brassicae(L.), its parasites and hyperparasites in England

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Abstract
Summary: Studies on populations ofBrevicoryne brassicae(L.), its parasites and hyperparasites were carried out by actual counting in the sprouts field and by sticky and water traps.B. brassicaewas found to be attacked by one primary parasite,Diaretiella rapae(McIntosh), which in turn is parasitized byAlloxysta brassicae(Ashm.),Asaphes vulgarisWalker,A. suspensus(Nees),Pachyneuron minutissimum(Förster) andDendrocerus carpenterii(Curtis). The aphid population in the field was started by immigrant alates which were found flying too early to be synchronized with the sprouts plants. SimilarlyD. rapaewas not synchronized with the aphids although many individuals could have been carried into a plot through parasitized immigrant alates, of which less than 30% were found parasitized. Because of high hyperparasitism (especially byA. brassicae)D. rapaewas not able to maintain a high rate of parasitism to curb the aphid population growth. The maximum percentage mummies being 27.8%, while the maximum, percentage parasitism being 56.6% recorded only during the early 1974 season (mean=12.9%). The decline of aphid population from September onwards was largely due to the cold weather, Syrphid predation and occasionally fungal attack. The high rate of hyperparasitism byA. brassicaeis attributed to its better synchronization withD. rapae. The mean percentage of parasite that emerged from mummies collected during 1973–74 wereD. rapae31.3%,A. brassicae64.3%,A. vulgarisandA. suspensus4.3%,D. carpenterii0.2% andP. minutissimum0.1%.