Release and evaluation of a laboratory-selected, pyrethroid-resistant strain of the predaceous mite Metaseiulus occidentalis (Acari: Phytoseiidae) in southern Oregon pear orchards and a Washington apple orchard.

A laboratory-selected strain of the spider mite predator Metaseiulus occidentalis (Nesbitt) was released into two southern Oregon pear orchards and a Washington apple orchard in June 1980. The permethrin and organophosphorus insecticide (OP)-resistant predators became established in all three orchards, overwintered, and survived repeated permethrin applications during 1980 and 1981. By summer 1981, the predators had spread into adjacent trees in release areas in both states. Foliage damage was substantially reduced compared with trees lacking the permethrin-OP-resistant predators in Oregon in 1981. in Washington in 1980, spider mite populations were significantly lower and M. occidentalis populations significantly higher on release trees than on control trees. In 1981, the frequency and rates of permethrin applications caused high mortality of the resistant M. occidentalis and high spider mite densities developed on all sampled trees. Permethrin resistance levels remained high in the orchard populations after two field seasons, despite the polygenic nature of this resistance. Future releases should be considered, especially where releases can be made into each tree and selective acaricides can be used to manage spider mite-predator densities until a more stable relationship occurs.