Migration of parasitoids (Hymenoptera: Braconidae) of cereal aphids (Hemiptera: Aphididae) between grassland, early-sown cereals and late-sown cereals in southern England

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Abstract
The dynamics of migration of the parasitoidsAphidiusspp. between grassland and cereal fields of varying sowing dates was investigated in southern England in 1980 and 1981. Using baffled water traps, direct evidence of migration ofA. rhopalosiphiDe Stefani Perez females from grassland to winter wheat was obtained in June 1980, suggesting that the early-maturing grassland aphidMetopolophium festucae(Theobald) can act as a reservoir of cereal aphid parasitoids. In 1981, estimates of parasitoid production in cereal fields containing the aphidsSitobion avenae(F.),M. dirhodum(Walker) andRhopalosiphum padi(L.) were compared with actual numbers to calculate net immigration and emigration. Barley and early-sown wheat (drilled before 14 October) proved to be significant sources ofAphidiusspp., with one wheat field generating sufficient parasitoids in May to account for immigration into about 25 late-sown fields. Parasitoid migration as larvae inside aphid alatae was apparently of considerably less significance than that by adults, which were predominantly female. The advantages of early drilling of cereals to form ‘semi perennial’ reservoirs of beneficial insects is discussed and weighed against the corresponding disadvantages, especially an increased risk of barley yellow dwarf virus (BYDV).