An Ecological Study of Phytoseiid and Tetranychid Mites on Walnut in Tulare County, California

Abstract

Mite populations were monitored for a 3-yr period in an 8 ha walnut orchard which, except for a 2 ha corner, was sprayed once annually with azinphosmethyl for codling moth, Laspeyresia pomonella (L.). European red mite, Panonychus ulmi (Koch), reached maximum densities in July each season and was generally highest in the unsprayed block. Twospotted spider mite, Tetranychus urticae Koch, was never abundant. Typhlodromus occidentalis Nesbitt was the prevalent phytoseiid mite species on the unsprayed trees, and showed the greatest numerical response to increases in spider mite populations. Typhlodromus citri Garman and McGregor was generally the most abundant phytoseiid on the sprayed trees. Neither of these 2 species was severely reduced by the azinphosmethyl sprays. Typhlodromus caudiglans Schuster and Amblyseius hibisci (Chant) generally comprised a small proportion of the phytoseiid population, and both were adversely affected by azinphosmethyl sprays. Hanging walnut husks and coverings of dead scale insects were major overwintering sites for T. citri and T. occidentalis , respectively. In another unsprayed orchard surveyed in 1974, twospotted spider mite populations increased to more than 30 mites/leaflet. Here, European red mite and phytoseiids were scarce, and the sixspotted thrips, Scolothrips sexmaculatus (Perg.), was considered the major predator of twospotted spider mite.