SPIDER-MITE (ACARI, TETRANYCHIDAE) INFESTATION RATE AND INITIATION - EFFECT ON COTTON YIELD

A 2-yr study was conducted to investigate the effect of spider mites, Tetranychus spp., on cotton (Gossypium hirsutum L.) in the West Central San Joaquin Valley of California. Results of each year were analyzed by spline (i.e., segmented) regression. In 1982, following infestation of cotton at almost-equal-to 550 degree-days (DD) > 12-degrees-C after planting, significant reduction in yield did not occur until a critical infestation rate (change in the proportion of infested leaves) reached 0.121/100 DD, after which, yields were dramatically reduced. Results of spline regression analysis suggest that cotton is initially tolerant of spider mites, but once the infestation rate exceeds the critical rate, tolerance breaks down rapidly. This critical rate is fundamental to the economic threshold for spider mites on cotton, which is suggested to be 0.8 proportion infested, P(I), leaves. Plots treated with an acaricide at 0.8 P(I), did not reach the critical infestation rate and did not experience economic loss. In 1983, yield reduction was significantly greater with early infestations than with late infestations. Combining the 1982 and 1983 data into a single regression analysis showed that the critical infestation rate increased with later infestation initiation. In a supplemental analysis, infestation rates were weighted as a function of relative spider mite species abundance and species-specific damage. Weighted infestation rates did not significantly improve or reduce the fit of the regression function. A benefit-cost analysis indicated that once the spider mite population rate of increase exceeds the critical infestation rate, yield suppression is dramatic and sufficient to warrant management intervention.