AN ECONOMIC THRESHOLD FUNCTION FOR THE TWOSPOTTED SPIDER MITE, TETRANYCHUS URTICAE (ACARI: TETRANYCHIDAE), ON STRAWBERRIES

Abstract The growth and yield responses of “Totem” strawberries, Fragaria × ananassa, to 4 introduction densities of the twospotted spider mite, Tetranychus urticae Koch, were observed under field conditions in the lower Fraser Valley. At the high-density treatments T. urticae populations peaked at 65 mites/leaflet and declined during harvest. At the medium- and low-density treatments and in the controls, T. urticae had lower and later peaks. Two species of insect predators and 1 phytoseiid species preyed upon T. urticae. Berry weight and number were predicted (R = 0.97, 12 df) from the number of flowering stalks and accumulated mite-days. Yields increased with increasing numbers of flowering stalks and declined with increasing accumulated mite-days. A $1300 yield loss/ha was observed when 2500 mite-days accumulated between 15 May and 16 July. Fruit size was not affected by T. urticae. Yield equations were developed so that a mite sample 1–5 weeks before harvest can be used to predict dollars lost due to T. urticae. The data allow the grower to determine an economic threshold based on the number of weeks before harvest, mite sample precision, differences in environmental conditions relative to the present study, the control measure to be used, and the market price to the grower.