12 Population Dynamics

It is the abundance of many aphids that makes them such serious pests. Therefore, it is important for aphid pest management to have a good understanding of their population dynamics both in terms of theory and practice. Although this aspect of aphid biology is well studied, there has been a lack of long-term studies of the population dynamics of aphids living on herbaceous plants, including crops. This is because any single arable crop field supports only a small fraction of the shifting population in a region, and even a dramatic event there will have little or no impact on the regional population dynamics (Mackauer and Way, 1976). As a result, this chapter will draw on research on non-pest species where their study may provide insights relevant to crop pests. Not surprisingly, the pest status of aphids and political concern over the prophylactic application of pesticides has attracted the attention of modellers since the 1960s (Hughes, 1963; Hughes and Gilbert, 1968; Gilbert and Hughes, 1971; Gosselke et al., 2001). Attempts were made to forecast the abundance of aphids and propose expert systems to help farmers optimize prophylactic measures and minimize their costs (Mann et al., 1986; Gonzalez-Andujara, 1993; Ro and Long, 1999). These studies usually concluded that forecasting is a better strategy than either no control or prophylaxis, where yields are average and above (Watt, 1983; Watt et al., 1984). The advisory systems, however, did not receive general acceptance and disappointingly few forecasting systems are in use. Analysis of some of the existing models of aphid population dynamics reveals the reasons. For example, a model that describes the summer population dynamics of the Sitobion avenae (grain aphid) (Carter et al., 1982; Carter, 1985) was modified and extended to include the population dynamics of the aphidophagous predator Coccinella septempunctata (7-spot ladybird) (Skirvin et al., 1997a,b). It is claimed to give better predictions than the Carter et al. (1982) model, but there are few data against which it can be validated. The main weakness of the Skirvin et al. (1997a) model is that it gives the same prediction for identical initial conditions, which is contrary to what is observed in the field. Early models of the population dynamics of Myzus persicae (peach–potato aphid) (Scopes, 1969; Tamaki and Weeks, 1972, 1973; Tamaki, 1973, 1984; DeLoach, 1974; Taylor, 1977; Whalon and Smilowitz, 1979; Tamaki et al., 1980, 1982; Mack and Smilowitz, 1981, 1982; Smilowitz, 1984; Ro