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.

いくつかの種のハダニは葉の表面に絹糸で巣を作り，その内部で繁殖する（WN種）．それらの営巣パターンは捕食回避と関係すると考えられるが，捕食者相に与える影響の検証例はスゴモリハダニ属などの少数の種に限られている．本研究では，高知県香美市において，常緑性のアラカシに寄生するカシノキマタハダニ（Schizotetranychus brevisetosus Ehara）の捕食者相を調査した．発見した個体（n＝129）のうち，ハネカクシ類が38％，ハモリダニ類が26％となり両者で全体の半数を占めた．一方，ハダニのパッチでよく見られるカブリダニ科は全体の15％に過ぎず，そのうち多くが越冬中であった．また，卵捕食者のナガヒシダニ科（13％）を除き，他の捕食者はまばらであった．カシノキマタハダニは冬の間も成虫と卵が葉の上に残るが，1月と2月に活動する捕食者はいなかった．これまでの研究結果を踏まえると，本種の巣網や防衛行動が捕食者ごとに異なる効果を及ぼした結果，捕食者相が他種と異なった可能性がある．