Results of a four year field study on the optimal growth stage and the critical aphid population (economic threshold) for the control of cereal aphids (Hom., Aphididae) in intensely grown winter wheat in western Germany

The results presented are based on the analysis of 50 field experiments conducted from 1980 to 1983 in the Federal Republic of Germany (from Kiel to Mainz). Plot size was 25 square metres, with 4 replicates and randomized layout of variables. Keeping plots free of cereal aphid attack from different developmental stages of the wheat plant onwards, led to the following conclusion: the optimal stage to control cereal aphids in wheat is the end of flowering, if aphid numbers do not exceed one per head and flag leaf at the beginning of flowering, already. Subsequent experiments with a single application of Pirimicarb (150 g a.i./ha) led to the following critical threshold: if, at the average, at the end of flowering of winter wheat, at least one cereal aphid per ear and flag leaf is found, insecticidal control is considered to be economic. If this aphid number is already reached before the beginning of flowering, aphids should be controlled. When this critical threshold is used, nearly one third of wheat fields is sprayed unnecessarily with insecticides. But, as compared with a routine control, only 50 per cent of fields are sprayed. It is shown, that a higher critical threshold (2 aphids) bears a risk for the single farmer. Only if the wheat price should be lowered extremely (application costs not rising), a higher threshold might be economic. If cereal aphid control was missed during flowering of wheat, or in case of a late population build up, a control during milky ripeness is only economic when 8 aphids per ear and flag leaf are exceeded, and even then in 50 per cent of fields only. Numbers of aphids per ear and flag leaf and numbers of ears and flag leaves attacked by aphids are positively correlated (p 0.001). The critical threshold at the end of flowering is reached if 30 per cent of ears and flag leaves are attacked by aphids. At early milky ripeness the critical threshold of 8 aphids corresponds to 95 per cent of attack