Control of cereal aphids in winter wheat by natural enemies: aphid-specific predators, parasitoids and pathogenic fungi.
Winter wheat fields on two farms in West Sussex were sampled in 1980 and 1981 for cereal aphids and their natural enemies. The grain aphid, Sitobion avenae was present in all 19 fields examined, but in no case did the populations increase to densities liable to cause economic damage. The observations strongly suggest that aphid population growth was stopped by aphid-specific predators, hymenopterous parasitoids and fungal pathogens. In two fields in 1980, S. avenae population densities approximately equalled five aphids per ear at flowering, the threshold at which insecticide application is recommended in the UK, but numbers were then reduced by natural enemies, mainly aphid-specific predators. In three fields in 1981, S. avenae would probably have exceeded the spray threshold had natural enemies not intervened in late May.