The cereal aphids Rhopalosiphum padi (L.) and Sitobion a venae (F.) cause spread of Barley Yellow Dwarf Virus in autumn sown crops over the winter. Control is achieved by correctly timed insecticide applications, commonly synthetic pyrethroids. Polyphagous predators contribute to the natural control of these aphids. A field investigation into the effects of deltamethrin on polyphagous predators,using barriered plots, showed that natural control may be reduced due to the reduction in numbers of predators over the winter. Also, spring populations, which limit the growth of summer populations of aphids, may be reduced as larvae are most affected. The field dispersal of apterous R. padi was simulated in a computer simulation model based on changes in distribution along crop rows, and found to be between 0.6 - 1.3 m day. Analysis of leaves, using ELISA, confirmed spread of virus in the crop, with a maximum in January. A damage code based on symptom expression in the crop was devised, but was of use only as a guide to infection. Dispersal was observed by release of apterous R. padi in the centre of nineteen 1m² experimental plots of wheat. Dispersal showed a step-like relationship with mean daily temperature and an "activity threshold" at 7-9° C. Dispersal rates were much less than those obtained from commercial fields, possibly due to density-dependent mortality. Experiments in controlled environment rooms showed that apterae moved greater distances at temperatures above the "activity threshold", but other factors were involved. Observation of individual R. padi showed that apterae were capable of walking ∅.7m hour at 11°± 2°C. The implications of all the results on improving forecasting and integrated control of cereal aphids and BYDV are discussed.

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