Biological Control of the Two-spotted Spider Mite by the Resistant Strain of Phytoseiulus persimilis

Biological control experiments of Tetranychus urticae KOCH by use of the resistant strain (hereafter called DAS-strain) of Phytoseiulus persimilis ATHIAS-HENRIOT which was introduced from West Germany were conducted on cucumber plants cultivated in a vinyl house. Furthermore, a system simulation by micro-computer was designed to develop a realistic model of the predator-prey system under pesticide spraying conditions. Resistance level against fenitrothion in the DAS-strain of P. persimilis increased with the replication of the fenitrothion treatment. However, the level of resistance diminished to the level before the selection by the pesticide, when the interval of the selection was extended to about 4-6 months. Under chemical spraying conditions, i.e. 7 applications of fungicides and 4 applications of fenitrothion, the DAS-strain predacious mites effectively controlled the spider mite population when they were introduced at the ratio of P. persimilis female: T. urticae female=1:10. Furthermore, the predator could more effectively suppress the prey population when fenitrothion was first sprayed on the 7th day or 10th day after the predator introduction. In these two cases, the leaf damage index (LDI) did not exceed a value of 2.0 during the course of the experiments, although the LDI level at the time of the predator introduction was 1.3. There were two kinds of discrepancy between the experimental data and system simulation. One was concerned with the mite developmental speed dependent on the temperature, and the another with the loss of the predators at the time of introduction. When the value of the mean daily temperature decreased to 85% of the observed values and the number of predators introduced decreased to 80%, the model successfully simulated the processes and results of all the experiments. If the changes in the driving variables could be controlled in the future, this model may become useful to predict the effectiveness of the DAS-strain of P. persimilis as an agent for the biological control of spider mites under pesticide spraying conditions.