Abundance of aphids on cereals from before 1973 to 1977.
At Rothamsted in southern England from 1973 to 1977, alate cereal aphids (Sitobion avenae (F.), Metopolophium dirhodum (Wlk.), and in some years S. fragariae (Wlk.) and Rhopalosiphum padi (L.)) migrated to cereal crops during the last fortnight of May and during June. When temperature, rainfall and wind were favourable for aphid reproduction, as in 1975 and 1976, S. avenae and M. dirhodum increased rapidly on the leaves and stems until the ears appeared. The ears were then colonised by S. avenae alatae and apterae. Morphs of all species on the leaves, stem and ears were parasitised by Aphidiidae, chiefly Aphidius picipes (Nees), A. ervi Hal., A. urticae Hal. and A. uzbekistanicus Luzhetzki. These were also caught in emergence traps and were most numerous after the peak catches of alate aphids. Parasites were parasitised by hyperparasites, namely Dendrocerus carpenteri (Curt.), D. breadalbimensis (Kieff.), D. bicolor (Kieff.), D. laticeps (Hedicke), D. aphidum (Rond.), Asaphes vulgaris Wlk., Phaenoglyphis villosa (Htg.) (piciceps (Thoms.)), Alloxysta victrix (Westw.) and occasionally by Coruna clavata Wlk. Early parasitism in years when the weather conditions were unfavourable for rapid reproduction and development of aphids, as in 1973 and 1974, probably lessened aphid numbers. The main aphid predators (syrphid larvae, coccinellid larvae and adults, chrysopid larvae and staphylinids) appeared at the end of June, later than the parasites, but when numerous, as coccinellids were in 1975 and 1976, they killed many aphids before the latter emigrated from the maturing crop. Neither parasites nor predators decreased the initial populations during June in years such as 1975 and 1976 that were climatically favourable for migration and multiplication. The relative numbers of aphids, parasites, hyperparasites and predators varied greatly during the 11 years 1967 to 1977. From a consideration of the effects of weather on the aphids, it is concluded that the accumulated temperature in day degrees above basal development temperature determines the potential population growth against which the effects of rainfall, wind, parasites, disease and predators can be examined.