Abstract
This chapter looks at the abundance on crops of several aphid species and what explanations for the patterns observed can be proposed from field-derived evidence. Although attempts over the years to understand the causes of population change of aphids on crops from field-collected data have provided good evidence for what underlies the one-, two- or three-peak cycles, all the methodologies can be criticized, and validation of the interpretations by experiment has proved difficult or impossible. The time-specific life-table approach has emerged as the best way to analyse population change, and it should perhaps now be revisited. Hutchinson and Hogg (1985) sought to improve Hughes's (1962) method in their studies with Acyrthosiphon pisum, replacing the calculation of 'potential increase rate' based on instar frequencies with the intrinsic rate of increase (estimated on a day-degree scale under field conditions in clip cages, i.e. in the absence of natural enemies). They found this gave a reliable estimate of the potential population growth rate.