Mutual interference between adult females of Galendromus flumenis (Acari: Phytoseiidae) feeding on eggs of Banks grass mite decreases predation efficiency and increases emigration rate

The Banks grass mite, Oligonychus pratensis (Banks) (Acari: Tetranychidae) causes significant damage to dates in California (USA), if not controlled. Studies are underway to develop biological control strategies against this pest in dates using the predatory mite Galendromus flumenis (Chant) (Acari: Phytoseiidae). In California date gardens, this predator is found in low numbers that are insufficient for the economic suppression of Banks grass mites, and our research aims to understand why it fails to keep up with prey densities. The hypothesis that prey density and predator interference interactively determine the predation efficiency of G. flumenis was tested. In addition, the effect of arena size and prey and predator density manipulations on the emigration rate of the predator was investigated. Our results indicate that the per capita predation rate of G. flumenis decreases steeply with increasing predator density due to mutual interference. Analysis of emigration data considering the arena size and predator numbers showed that the emigration rate of G. flumenis was higher from small arenas, and increased with increasing predator numbers. When emigration data were analyzed using prey and predator densities as independent variables, only the effect of predator density was significant, suggesting that higher predator density increases the emigration rate of G. flumenis. These results contribute to our understanding of the predator-prey interactions, and help in designing strategies for more efficient augmentative releases of G. flumenis.