Comparative Studies on Four Strains of Typhlodromus occidentalis. III. Evaluations of Releases of Insecticide Resistant Strains into an Apple Orchard Ecosystem12

Abstract

Insecticide-resistant strains of Typhlodromus occidentalis Nesbitt from Washington and Utah and a native susceptible strain which failed to persist in sprayed orchards were released into a sprayed orchard ecosystem at Oak Glen, near Yucaipa, California. Density and distributional parameters were followed throughout a summer and evaluations were made on the ability of these strains to withstand chemical treatments, adapt to the local environment, and control populations of Tetranychus medanieli McGregor at low densities under the conditions specified. Resistant predators from Washingtone and Utah numerically increased, persisted in the sprayed orchards, and demonstrated an enhanced ability to control populations of the pest species at lower levels when compared to the native susceptible strain, which only numerically responded to the host after insecticide residues had dissipated. Distributional data evidenced the respective numerical increases of each strain in time and confirmed the ability of T. occidentalis as an efficient predator. Trees on which the resistant strains were released (6 replicates/released strain) required no acaricidal treatments. The potential role of a resistant natural enemy was suggested and the prospect of long-term establishment of resistant mites was favorable from evaluations of the 1st year.