Monitoring and forecasting of cereal aphids in The Netherlands: A subsystem of EPIPRE

Cereal aphids have become increasingly important pests in Western Europe; during the last few years losses have often exce.eded 1000 kg/ha of wheat. Much of this damage is caused by secondary effects, since the actual loss of sap caused by aphid feeding is only of minor importance in plants growing under optimal conditions. Model calculations, confirmed by experimental measurements, show that the major reason for damage is honeydew produced by the aphids, which covers the epidermis, limits C02 diffusion, and probably promotes leaf senescence (Rabbinge and Vereijken, 1979; Carter and Dewar, 1981; Vereijken, 1979). This type of damage also explains why there is such a low correlation between aphid load, whether expressed as maximum number or as aphid days (integrated aphid numbers throughout the infestation period) and yield loss (r = 0.69, n = 21; Figure 15.1). This low correlation makes the definition of fixed damage thresholds impossible, and the prediction of expected yield losses possible only when more quantitative knowledge of the dynamics of aphid-host plant interrelations are included. Detailed studies of these damage effects have shown the complex nature of the yield losses, and have also demonstrated that the role of ants as mutualists of aphids is irrelevant, but that saprophytic fungi on the honeydew may have considerable effects. Three species of cereal aphid are usually involved: the English grain aphid (Sitobion avenae), the rose grain aphid (Metopolophium dirhodum), and the bird cherry-oat aphid (Rhosalosiphum padi). S. a venae is general1y .,considered the most important; it prefers to feed on wheat ears, where it interferes with the translocation of assimilates to the kernels. In 1978 and 1979, however, cereal losses exceeded 1000 kg/ha, caused mainly by large numbers of M. dirhodum, whileS. avenae was uncommon. ·