Biological Control of the Two-Spotted Spider Mite on Strawberry in Southern California1

Abstract

Biological control of the two-spotted spider mite, Tetranychus urticae (Koch), on strawberry was investigated in southern California during 1965. Mass releases of Phytoseiulus persimilis Athias-Henriot were made at the equivalent rate of 160,000 and 320,000 per acre at 2 different timings in 2 blocks of 5 plots each. Sanitation (pruned leaves removed from field) was practiced in 1 block and not in the other. Population trends of prey and predators were followed at weekly intervals throughout the study. The two-spotted spider mite population reached a peak May 11, averaging 21 mites per leaf in the block where sanitation was practiced and 129 per leaf where it was not. The highest population in a plot was 307 mites per leaf with 100%, of the weekly infested. P. persimilis reached its highest population in the same plot at 8 per leaf on May effectively controlled the two-spotted spider mite where they were released early at 320,000 per acre before the latter species reached I mite per leaf. Nine species of insect predators and several phytoseiid mites known to prey on spider mites were recorded. The six-spotted thrips, Scolothrips sexmaculatus (Pergande), was the commonest predator. Native predators had almost eliminated the two-spotted spider mite population in the check plots by the end of June. The average number of miles per leaf was found to be a better criterion than percentage of leaves infested for timing the release of P. persimilis for control of the spider mite. No definite correlation was found between strawberry yield and spider mite infestations. Abnormally cool, rainy weather for the area during early April temporarily adversely affected prey and predation.