Understanding the impact of a pest species on a particular crop is critical for the success of a pest management program. Field studies were conducted to determine the effect of the twospotted spider mite, Tetranychus urticae Koch, on marketable yield of strawberries during the 2008/2009 and 2009/2010 growing seasons. Low, medium, and high mite infestation levels were established by initial inoculations of 5, 10, and 20 twospotted spider mites per strawberry leaf, respectively. A control treatment maintained at near zero mites through applications of an acaricide, bifenazate (Acramite 50 WP), was also included. Weekly records of motile twospotted spider mites were obtained over 13 and 16 wk during the 2008/2009 and 2009/2010 growing seasons, respectively. Degree-days and weather parameters were monitored to determine their effect on mite population. In addition, mite-days were calculated for each year from weekly mite counts to determine the effect of mites on marketable yield of strawberries. In both years, twospotted spider mite population increased throughout the growing seasons. More degree-days were accumulated during the 2008/2009 growing season, and mite population was higher in 2008/2009 than in 2009/2010. Mite population density per leaf increased up to 278 motiles per leaf in 2008/2009 growing season as compared with 137 in 2009/2010 within the high-infestation-level treatment. The divergence in mite population between the two growing seasons was attributed mainly to temperature differences between the two seasons that affected mite population development and establishment. During both growing seasons, the high mite infestation level had lowest marketable yield. A negative correlation between cumulative mite-days and harvested marketable yields was detected in both seasons, but it was only significant during the 2008/2009 growing season. Strawberry yield reduction was detected when plants attained 80 mites per leaf in 2008/2009 and 50 mites per leaf in 2009/2010 within the high mite infestation treatment. Factors that affect mite population establishment and management for twospotted spider mites on strawberries are discussed.
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Vol. 106 • No. 4