Quantifying aphid predation: the mealy plum aphid Hyalopterus pruni in California as a case study.
Generalist insect predators provide an important ecosystem service in the regulation of agricultural pests. However, the role of predation in the dynamics of aphid populations has been difficult to quantify as any change in aphid density results from a balance between two opposing processes, aphid recruitment through reproduction and aphid mortality through predation. Our goal in this study was to determine the degree to which aphid consumption by naturally occurring densities of aphidophagous predators is responsible for observed seasonal changes in aphid densities. We quantified consumptive predation using a mechanistic approach that incorporates the balance between aphid recruitment and mortality. We used the mealy plum aphid Hyalopterus pruni in Prunus domestica plum orchards as a model system. Our approach was to compare two quantities: a demographic-based estimate of predation and an observation-based estimate of predation. The demographic-based estimate used a simple model of aphid population growth and predation. It estimates the mortality that must have occurred to give rise to observed changes in aphid densities between sample dates. The model uses independent estimates of aphid population growth rate in the absence of predation. The observation-based estimate combined field data on predator densities in plum orchards with stage- and species-specific per capita predation rates estimated from field observation. Predation may have reduced the rate at which mealy plum aphid populations increased early in the season and rebounded from mid-season declines, but was not sufficient to prevent high aphid densities or to accelerate rates of decline later in the season. Moreover, predation at naturally occurring densities did not appear sufficient to drive the directional changes in mealy plum aphid densities at any point through the season. Synthesis and applications. Our study shows that natural densities of generalist predators can be too low to provide effective biological control services for an aphid pest. Generalist aphid predators could provide more effective control if early-season densities were increased through augmentative releases or encouragement of crop colonization, but the commercial viability of these options remains to be tested.