During migration the spatial distribution of aphids must become diffuse because migration is dependent on the wind. Mortality during host finding is of the order of hundreds, possibly thousands to one and the subsequent capacity for reproduction, once a migrant is established, is correspondingly great, so that the numbers and distribution of aphids are perhaps more a function of distance travelled, efficiency of host-finding, and reproductive rates than of subsequent mortality. There has yet been no evidence of any resulting geographical pattern in the epidemiology of aphids, nor any field assessment of the relative contribution made by the environment and by the individual behaviour, either at morph or species level, to the ultimate spatial distribution of populations. It therefore remains to be seen if aphids such as Myzus persicae (Sulz.), which feed mainly on annual plants, are as effective in finding and exploiting their spatially mobile resources as their highly sophisticated resource-searching adaptations might lead one to expect (Kennedy & Stroyan 1959; Taylor 1965, 1975). There are three seasonal cycles of population growth and redistribution in Myzus persicae in Great Britain, in common with many other aphids, and there is a progressive seasonal change in the morphology of migrants, although this is less pronounced in anholocyclic populations (Woodford & Lerman 1974). The migratory behaviour may be supposed to cycle correspondingly (Shaw 1973a, b). Control of migrant production and migratoriness originates in the environment, acting in complex ways through differential, temperature-dependent, plant and aphid population growth rates which in turn affect plant condition and density-dependent contact between individuals. In addition, temperature and light thresholds for take-off (Dry & Taylor 1970), and for continued flight, limit the occasions when migration can occur (Berry & Taylor 1968) but when it does, distances of tens or even hundreds of kilometers are commonly expected (Johnson 1969). These distances could thus have seasonal cycles, differing between the three migrations because of differences in weather, population density and individual behaviour, and this could affect gradients of distribution and the subsequent exploitation of resources. When the aphid is host to a parasitic organism, such as a virus, the epidemiology of the parasite is even more difficult to analyse. The analytical problems stem largely from the vast scale of the movements of aphids and the difficulty of following them in flight, but also from the different dimensions involved in the analytical process. The controlling variables for change in the overall population of the parasite are usually arrived at by a temporal modelling approach, often involving some kind of regression analysis (Watson & Healey 1953; Varley & Gradwell 1960). In contrast, it is the spatial distribution of the plant host that conditions the movements of the vector, whilst the timing of migration and differential survival between migrations are determined directly or indirectly by the