Effects of Releases and Varying Infestation Levels of the Twospotted Spider Mite on Strawberry Yield in Southern California1

Abstract

Effects of releases and varying infestation levels of Tetranychus urticae Kock on fruit yield were studied on strawberries in southern California. In the winter planting, fruit yield and size were significantly higher in the 20–25 density treatment level (DTL) plot (seasonal accumulation of 1,298 mite-days/leaflet) than that in the 90–100 DTL plot (6,371 mitedays/leaflet) in May only. There was no difference in fruit yield or size between the early release and untreated check plots (seasonal accumulations of 24,734 and 23, 135 mite-days/leaflet, respectively), but fruit yields in these 2 plots were significantly lower than that in the 2 DTL plots in May and June. In the summer planting, seasonal accumulations of 1,135 and 2,950 mite-days/leaflet in the 20–25 and 90–100 DTL plots, respectively, did not result in significant differences in fruit yield or size between the 2 levels. Fruit yield and size in May and fruit yield in June were significantly lower in the early release plot than that in the late release plot (seasonal accumulations of 11,343 and 8,168 mite-days/leaflet, respectively). Fruit yield in the early and late release plots in May and in the early release plot in June was significantly lower than that in the check plot (seasonal accumulation of 9,267 mite-days/leaflet). Also, fruit size in the early release plot in June was significantly lower than that in the check plot. Eight insectan predators were present with the cecidomyiid, Arthrocnodax occidentalis Felt, being the predominant species followed by the sixspotted thrips, Scolothrips sexmaculatus (Pergande).