Prey preference by Metaseiulus occidentalis (Acari: Phytoseiidae) and the role of prey aggregation

Abstract

Abstract The phytoseiid predator Metaseiulus occidentalis is an important biological control agent for the Pacific spider mite (PSM) Tetranychus pacificus and the Willamette spider mite (WSM) Eotetranychus willamettei, two economically important spider mite species in California vineyards. Field observations indicated that M. occidentalis is more efficient against PSM than it is against WSM. This paper examines prey preference by M. occidentalis, as it relates to the spatial aggregations of PSM and WSM, to explain differences in M. occidentalis's efficiency on PSM and WSM in vineyards. Prey preference by M. occidentalis was studied in laboratory (individual) and greenhouse (population) experiments. In both experiments, M. occidentalis showed an overall preference for PSM over WSM; however, this preference was dependent on the relative abundance of available prey. An excess of the more abundant prey always was consumed. On the basis of theoretical and experimental consideration of the response of predators to aggregated prey, we conclude that prey preference by M. occidentalis appears to be related to differences in its prey's spatial aggregation. PSM was more aggregated than WSM on grape leaves in the greenhouse, and therefore was generally the more preferred prey. Furthermore, changes in M. occidentalis's prey preference appear to be related also to changes in the relative aggregation of PSM and WSM. Changes in relative aggregation are strongly correlated with changes in prey preference by M. occidentalis. Also discussed is the need for predation experiments to complement experiments on the response of predators to prey cues, especially in preintroductory screening of natural enemies.