We investigated the effects of weed mowing height on the occurrence of phytoseiid mites on white clover plants in apple orchards in Northern Japan. First, we investigated the species composition of phytoseiid mites in four orchards with different weed mowing heights and pesticide spray programs, as well as a playground without trees or pesticide spray. We found that Amblyseius tsugawai females were abundant on the white clover when weeds were mowed at heights ≧ 80mm, in contrast, Neoseiulus makuwa females were dominant when weeds were mowed at 10mm, and few Amblyseius tsugawai females observed at this height. The type of pesticide spray program applied seemed to have little effect on phytoseiid species composition. In the apple orchard where A. tsugawai females had been abundant, we further investigated the effects of mowing weeds at three height levels: 80, 40, and 10mm. This resulted in a significant decrease in the abundance of A. tsugawai at mowing heights of 40 and 10mm compared with their abundance at 80mm. In additional experiments, we found that A. tsugawai females predated more on the eggs and females of Tetranychus urticae than did N. makuwa females. Taken together, our results suggest that mowing weeds at higher heights in apple orchards promotes the conservation of A. tsugawai, which is an effective natural enemy of T. urticae.
Several species of spider mites construct silk nests on leaf surfaces wherein they develop and reproduce (web-nesting species). Nesting patterns and behaviour of spider mites are related to predator avoidance, but the study of their influence on predatory fauna has been limited to species of a few genera. The present study investigates the occurrence of predators of Schizotetranychus brevisetosus, which make web nests and attack some predators, on evergreen oak in Kochi Prefecture, Japan. Of the total individual predators observed (n = 129), the highest proportion (38%) consisted of rove beetles Holobus spp. (Coleoptera: Staphylinidae), which are nest intruders and prey on mites at all stages of development. The second most abundant predators (26%) were Anystis spp. (Acari: Anystidae), which capture mites outside the nest. The common predators Phytoseiidae spp. (Acari) accounted for only 15% and were mostly found in hibernation. Other predators were rare, except for egg-eating Agistemus spp. (Acari: Stigmaeidae) (13%). The findings suggest that web nests and anti-predatory behaviours may bias the predatory fauna toward species that can cope with these anti-predatory strategies.