The current spotted wing drosophila, Drosophila suzukii Matsumura (Diptera: Drosophilidae) management strategy relies on routine application of insecticides during the fruit ripening and harvesting period. The amount and frequency of insecticide spray needed to maintain continuous protection of fruits may depend on a variety of factors including the level of rainfall. Studies were conducted to determine the effects of simulated rainfall (0, 12.5, 25 and 37.5 mm) and adjuvant-Nu Film (with and without) on the residual efficacy of commonly used insecticides against D. suzukii in blueberries during the fruiting season of 2014 and 2015. Spray applications were made using an air-assisted backpack (2014) or airblast (2015) sprayer. Insecticide spray residues were allowed to dry for ~2 h before simulated rainfalls were applied using central pivot irrigation system. Semi-field bioassays were conducted to assess mortality at 1, 3 and 7 days after treatment (DAT). Results showed that simulated rainfall caused a significant reduction in D. suzukii mortality in all chemical treatments, particularly at 1 and 3 DAT in the 2014 study, and at 1 DAT in the 2015 study. In general, higher rainfall resulted in lower mortality in 2014, however such a clear trend was not observed in 2015. Addition of adjuvant, Nu Film 17, significantly increased mortality at 3 DAT in the zeta-cypermethrin treatment with 25 mm of simulated rainfall (2014). In 2015, the addition of Nu Film P significantly increased mortality at 1 DAT in the spinetoram and spinosad treatments with 12.5 mm, and in the malathion treatment with 25 mm of simulated rainfall. These results suggest that although frequent rainfall incidents increase the need for more frequent spray of insecticides, addition of adjuvants helps to prolong the residual efficacy of some insecticides.

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