Cereal Aphid Populations: Biology, Simulation and Prediction.
The large area of land devoted to cereal crops, with nearly 40% of all arable land under wheat, and the apparent increase in the incidence of cereal pests and diseases over the last twenty years justify a study of cereal pests. In terms of hectarage wheat is one of the most important crops in the world and any loss of yield caused by pests has serious consequences, both locally and world-wide. In Great Britain cereal aphids were not considered as important pests until 1968, when they reached very high levels on wheat (Fletcher & Bardner, 1969). George (1974, 1975) and Kolbe (1969, 1970) have shown that cereal aphids can cause considerable losses of yield in some years. Their abundance, however, varies from year to year (Carter et al., 1980; Rabbinge et al., 1979) and from place to place (George & Gair, 1979). For an effective advisory service knowledge of loss of yield relative to aphid density and the growth of aphid populations on cereals are needed. The latter is dealt with in this study; the former is discussed by Rabbinge et al. (1981). To predict cereal aphid outbreaks it is necessary to understand what causes the spatial and temporal differences in abundance.
Over the last decade considerable changes have occured in the cultivation of wheat in Western Europe. High sowing densities, split nitrogen dressing and top dressing at flowering have resulted in crops that remain suitable for cereal aphids up to the end of crop growth. To prevent losses of yield there has been a marked increase in the amount of biocide (herbicides, fungicides and insecticides) applied to wheat crops. There has also been a tendency to use insurance spraying, i.e. the regular application of biocides at particular crop developmental stages without verifying the presence of pests or disease. This causes an overuse of pesticides, which increases costs and reduces profit per hectare (Rijsdijk et al., 1981) and can result in the development of resistance to pesticides as has been recorded for orchard pests (Helle & Van de Vrie, 1974), and may increase the incidence of secondary pests and diseases (Baronyovits, 1973; Potts, 1977). The improvement in growing conditions has resulted in wheat crops of 9,000-10,000 kg ha-1. This has also had consequences for the development of pests and diseases, as these wheat crops suffer relatively more from pests and diseases than poorer crops (Rabbinge and Rijsdijk, 1982). In the absence of more effective means of controlling the pests and diseases of wheat, pesticides will be used even more frequently in the future. Zadoks et al. (in press) have used a systems approach to predict the incidence of wheat pathogens in the Netherlands (called EPIPRE = EPIdemic PREvention). Those farmers who use this system obtain yields similar to farmers who do not, but with fewer sprays, and hence lower costs.Throughout Western Europe cereal aphids are serious pests, and in the early seventies studies on their epidemiology were started in several countries. Several simulation models of cereal aphid populations have been developed, for example that of Rabbinge et al. (1979), which was developed for use in conjunction with EPIPRE in the Netherlands. The model presented in this book was developed for Norfolk, England.After Lincolnshire, Norfolk is the most important cereal growing county in England and Wales (Anonymous, 1980). Although more barley is grown than wheat, there were still over 85,000 ha under wheat in Norfolk in 1979. This was a major factor in the decision to work on cereal aphids at Norwich. From 1976 to 1980, with the notable exception of 1979, when Metopolophium dirhodum was the most common aphid, the English grain aphid, Sitobion avenae, has been the most numerous cereal aphid species on wheat in Western Europe, and as a consequence most research has been done on this species.