A field evaluation of different criteria for determiningthe need to treat winter wheat against the grain aphid Sitobion avenae and the rose‐grain aphid Metopolophium dirhodum

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Abstract
Summary: The economics of control of grain aphid (Sitobion avenae) and rose‐grain aphid (Metopolophium dirhodum) were investigated in a series of experiments carried out across England in 1988 and 1989. Sprays of pirimicarb or dimethoate were applied at three growth stages and yield effects measured. Aphid infestations were determined by a range of different methods, to evaluate the value of each for decision making.Aphids overwintered in crops in mild conditions in both seasons, resulting in early population build‐up and decline in many cases. Aphicide sprays applied during the booting stage increased average yields by 0.26 t/ha. Profitability of the treatment strategies considered was not greatly enhanced by selective spray application related to aphid assessment at this stage. Delaying decision making to the flowering or grain‐filling stages resulted in reduced profitability. A poor correlation between aphid numbers and crop yield response to sprays was found. This was shown to be due to differences in subsequent aphid population development and in aphicide efficacy. Grain quality was affected only by high aphid infestations which caused large yield reductions.