Organisms allocate their resources either to growth or dispersal. It is, therefore, of interest to understand what determines the relative benefits of the two options. The specific case to be addressed in this chapter is dispersal in tree-dwelling aphids. It is an important component of the reproduction of each clone: the evolutionary individual.

For a long time it was speculated (Taylor, 1974a), and now appears that dispersal in aphids is risky, with fewer than 1 in 100 locating a host (Ward et al., 1998). The incidence of dispersal is highly variable, with local populations of some species of tree-dwelling aphid declining by up to 63% per week in late summer mainly as a result of mass emigration, whereas others have low dispersal rates. Host specialists living on long-lived hosts such as trees should stay put because density-dependent effects are rarely likely to reduce replacement to fewer than 1, and only 1 in 100, or less, will survive if they disperse. Thus it is surprising that many species appear to show a marked tendency to emigrate.

Emigrants from one local population could be immigrants to another local population. Therefore in attempting to define the place a species occupies it is important to consider the dispersal and/or migration requirements of the organism in question (Andrewartha & Birch, 1984; Huffaker et al., 1999).