Long-established populations ofTyphlodromus pyriScheuten were less common in plots in 1993 with denseZetzellia mali(Ewing) than in plots with sparseZ. mali. Also, newly releasedT. pyrihad more difficulty colonizing plots withZ. malithan without, but less difficulty thanMetaseiulus occidentalis(Nesbitt) had had in 1992. Newly releasedZ. malidid not establish well in a plot that hadT. pyrionly, butZ. maliseemed to be displacingT. pyriafter 4 yr in some mixed-species release plots ofM. occidentalisandT. pyri. After immigration from nearby vegetation to apple, colonization byAmblyseius andersoni(Chant) was less in azinphosmethyl-treated plots than in similar untreated plots. Colonization byA. andersoniwas greatest in plots with fewT. pyrior Z.malior both but some occurred in all plots not sprayed in 1993 (those originally receiving no releases, single or mixed-species releases ofT. pyri, andM. occidentalisin 1990). Colonization byA. andersoniwas greatest whereM. occidentalishad been displaced in 1991 or 1992 byZ. mali.Results are discussed in relation to long-term biological control and more biologically based integrated pest management.