Impact of twospotted spider mites (Acari: Tetranychidae) on marketable yield of field-grown strawberries

The twospotted spider mite (Tetranychus urticae Koch) is the most damaging mite pest in strawberries. Damage to the leaves has been reported to negatively affect strawberry yield, but this have not been quantified in Florida. The objective of our study was to evaluate the effect of T. urticae on the marketable yield of strawberries. In field experiments, three mite infestations levels (high, medium and low) were evaluated alongside a control to determine their effect on strawberry yields. Experimental designs were completely randomized block and each treatment was replicated 5 times in 2008 and four times in 2009. Initial T. urticae inoculations were aimed at achieving infestation levels of 5, 10 and 20 (low, medium, high) mites per leaf respectively. Tetranychus urticae motiles were recorded once every week for 13 and 16 weeks in 2008 and 2009, respectively. Accumulated mite-days were calculated for each year. There were significant differences between treatments in both years for accumulated mite-days. In 2008, up to 9042 mite-days were accumulated while in 2009 there were 2645. Mite populations were greatly affected by low temperatures experienced in 2009. In 2008, the control and low mite infestation treatment had significantly higher marketable yields than the medium and high mite infestation levels. Marketable yield of strawberry began to decrease from day 43 and 98 in 2008 and 2009, respectively after heavy mite infestations. On those days, the accumulated mite-days reached 3017 in 2008 and 1938 in 2009. Overall, there was a negative correlation between accumulated mite-days and harvested marketable yields.