Occurrence of Spotted Wing Drosophila (Drosophila suzukii) in Wild Berries in Forested Areas of Marquette and Keweenaw Counties, Upper Peninsula of Michigan

Spotted-wing Drosophila (SWD, Drosophila suzukii Matsumura, Diptera: Drosophilidae) is an invasive fly that targets unripe soft-fruits causing premature ripening, dehiscence, and rotting. While the presence of SWD in agricultural systems is well-documented, much less is known about the host range of SWD in wild areas, especially in more northern latitudes where colder temperatures and short summers are thought to restrict their life-cycles and limit population growth. In 2019 and 2020, we collected wild berries from two counties in the Upper Peninsula of Michigan, with long, cold winters, to determine the prevalence, host range, and seasonal/temporal variation of SWD. We found SWD in five of seven different plant families surveyed but we restricted statistical analyses to three families in which we predominantly collected from and in which SWD larvae were most common: Caprifoliaceae, Ericaceae, and Rosacea. In general, average numbers of SWD per berry were highest for Rosaceae species and this pattern is likely attributed to the thin cell walls and poly-drupelet structure of their fruits that is known to enhance sustained infestation. SWD infestation intensity also generally increased throughout the season for all host families; a pattern most likely due to the ripening of fruits and associated increase in sugar content as the season progresses. Lastly, although we found that SWD abundances, on average, were higher in 2020 than in 2019 and in berries collected from Keweenaw versus Marquette County, these patterns could be due to sampling artefacts (e.g., unequal host sampling) rather than to climate differences between years and/or sites. To better assess future consequences that SWD infestations may have for humans, wildlife, and plant communities in non-agricultural northern temperate ecosystems, we suggest that more data is needed on how temporal and spatial climate variability effects SWD infestation patterns in these systems.