Optimization of a Larval Sampling Method for Monitoring Drosophila suzukii (Diptera: Drosophilidae) in Blueberries

Managing spotted-wing drosophila, Drosophila suzukii (Matsumura), in fruit crops is complicated by the unreliability of currently available traps for monitoring adult flies, combined with the difficulty of detecting larval infestation before fruit damage is apparent. A simple method to extract larvae from fruit in liquid, strain the solution, then count them in a coffee filter was developed recently for use in integrated pest management programs. Here, we present a series of experiments conducted to improve fruit sampling by making it faster, less expensive, and more accurate. The volume of blueberries sampled (59-473 ml) did not significantly affect the detection of second and third instars, but we found that 118-ml samples were best for detecting the smallest larvae. These small instars were more detectable when berries were lightly squeezed before immersion, whereas larger instars were similarly detectable without using this step. We also found that immersing fruit for 30 min was sufficient before counting larvae, and similar numbers of larvae were found in the filter using room temperature water rather than a salt solution. The process of filtering, detection, and counting larvae took only 2-4 min per sample to process, depending on larval density. Using a microscope to count the larvae was consistently the best approach for detecting D. suzukii larvae. Based on these results, we discuss how fruit sampling can be streamlined within IPM programs, so growers and their advisors can improve control and reduce the cost of monitoring this invasive pest.