Genetics, gene flow, and distribution of pesticide resistance traits were examined for organophosphate (OP) resistance in three beneficial phytoseiid mites. Levels and genetics of OP resistance in Amblyseius andersoni were examined first. Laboratory strains from Italy and Oregon, USA, were compared in susceptibility to insecticides used in western Oregon fruit crops. The Italian strain was 80-100 times more resistant to the OPs azinphosmethyl, diazinon, malathion, and phosalone, as well as carbaryl, a carbamate. Significant differences were not found between strains for endosulfan or fenvalerate. Using backcross analysis, response of F1 hybrids to azinphosmethyl indicated OP resistance was semidominant. Through novel statistical analysis, backcross of F1 to parent strains revealed resistance was polygenic, with at least two loci. Reciprocal crosses demonstrated the presence of maternal effects, with increased variation associated with progeny of Oregon females. In the next set of experiments, electrophoresis of allozymes was used to estimate gene flow for Typhlodromus pyri. Ten populations from two apple growing valleys of Oregon were compared. Subpopulations were collected from in and around commercial apple orchards. Four loci unaffected by pesticide use were examined. FST was calculated at 0.115, and Nm as 2.08. No allelic patterns could be discerned for populations among or within valleys; however, more variation was present for mite populations within valleys than between them. Some inbreeding was found within populations. While from dispersal studies one would conclude T. pyri is nondispersive, allozymic analysis indicates there is moderate gene flow. Factors affecting OP resistance distribution in T. pyri and Metaseiulus occidentalis were examined. A diagnostic concentration of azinphosmethyl was used to determine OP resistance frequencies for populations of each species, collected in and near commercial apple orchards in two valleys. OP resistance in T. pyri populations was localized: mites from 10 m or more outside orchards were OP susceptible, while those within orchards were resistant. This indicated limited gene flow. All M. occidentalis populations were resistant, indicating a regional resistance pattern and high gene flow. Factors which were not significant in the distribution of OP resistance were: valley, degree of orchard isolation, host plant, and seasonality. / Graduation date: 1994