Detection of Spotted-wing Drosophila (Drosophila suzukii) in Indiana blueberry orchards using degree-day models and molecular assays

Spotted-wing Drosophila (SWD), Drosophila suzukii (Matsumura), is an economically-important pest of small fruits worldwide. Currently, timing of management is based on morphological identification of adult flies captured in baited monitoring traps; however, distinguishing SWD from other native drosophilids in traps is a time-consuming process that requires magnification. And a degree-day model that could help small fruit growers understand and predict the seasonal activity of this pest has not been developed for Indiana. Due to the low tolerance for maggots in fruit market, most small fruit growers rely on intensive, insecticide applications on a calendar-based schedule without guidance on the activity levels of SWD. A total of 6,051 SWD adults were monitored weekly using commercial Scentry traps at three highbush blueberry orchards during May to August. I applied the published SWD developmental thresholds of 7.2 °C (lower) and 31.5 °C (upper) and the single-sine method to calculate accumulated degree days in the year of 2021 and 2022. A predictive model from two years of data at three locations exhibited an S-shaped curve, with 5%, 25%, and 50% of adults detected at ~907, 1,293, and 1,523 CDD, respectively. By examining infestations in three varieties, ‘Bluecrop’, ‘Blueray’, and ‘Elliot’, I found that blueberry infestation rate increased as the trap captures increased. The use of early-ripening highbush blueberry varieties can reduce infestation and regardless of variety, as berries became softer, the number of SWD egg scars in berries increased. DNA-based diagnostic methods, like loop-mediated isothermal amplification (LAMP), have the potential to improve SWD detection by replacing morphological identification with DNA-based identification. Positive results of the LAMP assay are based on a visible color change from pink to yellow when focal DNA is present. I tested the reliability of LAMP results using SWD DNA and then evaluated the sensitivity of LAMP in discriminating between SWD and two native drosophilids common captured in monitoring traps in Indiana, Drosophila affinis and D. simulans. I found the LAMP assay can quickly and accurately identify SWD with as little as 0.1 ng/μl of DNA. Following optimization, the assay also suggested success in discriminating between SWD and these two native species: it only requires an individual fly, DNA extraction is not necessary. By better predicting seasonal SWD activity and optimizing DNA-based diagnostics for this pest, this study can help improve the timely detection of SWD and the management in small fruit systems.