The importance of population structure in predicting pest population dynamics: A case study with Acyrthosiphon pisum

Whether natural enemies are able to prevent pest outbreaks critically depends on the population growth rate following the dispersal of pest insects into agricultural fields. Once pest populations exceed critical density thresholds natural enemies are no longer able to control pest populations (Tenhumberg 2004, Fagan et al 2005). In the early phase of population establishment populations exhibit transient dynamics because the population structure (relative proportion of juvenile stages to adult stages) deviates from stability, which can lead to transient dynamics that differ significantly from the stable-stage dynamics. Using the aphid, Acythosiphon pisum, as a case study I demonstrate that ignoring the stage structure following a dispersal event can result in a substantial underestimation of population growth of pest insects, and this transient amplification increases with improved growing conditions like higher temperature. For instance, a stage structured population model predicts that the population density 20 days after the arrival of a single adult aphid is 283 aphids, compared to a density of 94 aphids predicted from a model that ignores that the initial population structure deviates from stability.