Spatial aggregation and refugia of the mites Tetranychus urticae and Neoseiulus fallacis (Acari: Tetranychidae, Phytoseiidae) on hop

Three quantitative methods (Taylor, Iwao, and binomial) were used to estimate spatial aggregation patterns and refugia of the spider mite prey, Tetranychus urticae Koch and the phytoseiid mite predator Neoseiulus fallacis (Garman) on hop relative to the time of year, plant height, and presence or absence of predators. Using the Taylor methods, there was little change in aggregation of T. urticae with plant height, whether predators were released or not, and only a small change with season. The Iwao parameters indicated that spider mite colonies became larger but less clumped through the season and that in the presence of predators, colony size decreased and patchiness increased. The seasonal change in predator distribution as measured by the Iwao parameters closely reflected prey distributions with clump size increasing and clumps becoming less aggregated. Binomial data showed there was a refugium of hop leaves in which T. urticae did not occur and a refugium of spider mites colonies without predators. These refugia probably result from the combined effects of rapid growth of the hop plant and a spatial incongruence of predator and prey. The refugia may add to stability and persistence of predators and prey on hops. Implications of spatial patterns and refugia to biological control and sampling are discussed.