A fast and reliable larval sampling method for improving the monitoring of fruit flies in soft and stone fruits

The spotted-wing drosophila, Drosophila suzukii (Matsumura) (Diptera: Drosophilidae), threatens both the soft-skinned and stone fruit industry in Asia, Europe and America. Integrated pest management requires monitoring for infestation rates in real-time. Although baited traps are widely used for field monitoring, trap captures are weakly correlated to infestation rates. Thus, monitoring for larvae instead of adult flies represents the most reliable monitoring technique. Current methods for larval monitoring (e.g. dunk flotation) are both time-consuming and labor-intensive. In this study, we develop the sleeve method, a new method for monitoring larval infestation in strawberries through the inspection of fruits individually crushed within transparent plastic sleeves. Based on count data from both specialist and non-specialist observers, the estimation of larval infestation with the sleeve method is fast, precise and highly repeatable within and among all observers. Mean processing time is twice faster than previous methods and varies from 33 to 80 seconds per sample, depending on infestation levels. As the accuracy of the sleeve method decreases with infestation levels, we suggest ways to improve its accuracy by incubating fruits for 48h and calibrating data with exact counts. This new method is easier to use, faster and more cost-effective than previous monitoring methods. It could be used to monitor D. suzukii infestations in various fruit crops and is scalable to the farm-level for fruit growers or for academic research. Finally, the method represents a promising monitoring tool for effective management programs of D. suzukii and other insect pests of soft and stone fruits.