Effects of Different Infestation Levels of the Twospotted Spider Mite on Strawberry Yield in Winter Plantings in Southern California1

Abstract

Effects of different infestation levels of Tetranychus urticae Koch on fruit yield and size were studied in a winter planting of strawberries in southern California in 1979 and 1980. Seasonal accumulations of 168, 1,002, 1,332, and 2,129 mite-daxs per leaflet in the 0 to 5, 10 to 15, 20 to 25, and 90 to 100 density treatment level (DTL) plots, respectively, resulted in no significant differences in mean total fruit yield and size between treatment plots, but all plots were significantly different from the untreated control plot (25,206 mite-days per leaflet). Five, two, two, and one application of cyhexatin were required to keep the T. urticae populations below the 0 to 5, 10 to 15, 20 to 25, and 90 to 100 DTLs. respectively. In the untreated control plot, the T. Urticae population reached a peak on 22 April 1980, averaging 980.0 mites per leaflet. The predator population peaked on 6 May, averaging 6.1 predators per leaflet. Stethorus picipes Casey was the predominant predator, accounting for 43.8% of the total number (2,752) recorded.