Management Strategies for the Invasive Spotted Wing Drosophila

Mary Rogers, Assistant Professor, Andrew Petran, Postdoctoral Associate, UM Department of Horticultural Science and Steve Poppe, West Central Research and Outreach Center

Spotted wing drosophila (Drosophila suzukii, SWD) is an invasive fruit fly that has been present in MN since 2012. Female SWD have a serrated ovipositor and are able to infest intact, ripening fruit posing a significant risk to the U.S. fruit industry. SWD is a highly opportunistic pest with many attributes that contribute to its success as an invasive species, including a broad host range of cultivated and wild fruit species, rapid development times of only 10 days from egg- adult during summer months, and high reproductive rates –all of which contribute to rapid population growth under field conditions.  Crop losses have been reported as high as 40, 20 and 50 percent in blueberry, strawberry and raspberry, respectively. St. Paul Campus faculty and staff have reported negative economic effects on small fruit growers in MN. In 2016, they observed up to 100% damage in fall-fruiting raspberries in their small plots research trials on the St. Paul campus of the UMN (Mary Rogers, personal observation). Small fruit acreage in MN exceeds 1500 acres of raspberries, strawberries, grapes, and blueberries planted (USDA, NASS 2012; MN Dept. of Ag. 2016) with mid- season and fall-bearing blueberries and raspberries most at risk. One leading fruit and vegetable grower recently stated “since 2012, at least 25% of MN berry growers have either gone out of business or suspended the berry portion of their business due to SWD”.

Due to rapid reproduction and multiple overlapping generations, SWD populations can become devastatingly high in a short amount of time. In addition, there is a zero tolerance threshold for SWD in fresh fruit. Therefore, management techniques for SWD involve repeat applications of broad-spectrum insecticides. Weekly sprays may have deleterious ecological impacts on beneficial non-target species including pollinators, soil and water quality, contribute to insecticide resistance, and may even fail to adequately protect fruit from infestation. For example, weekly conventional insecticide rotations of two popular insecticides in fall-bearing raspberries still resulted in infestation levels of 60% likely due to high populations, immigration by new individuals, and/or inadequate spray penetration into the dense plant canopy.

Mesh netting for raspberriesAt the WCROC, we will investigate the efficacy of innovative and alternative management techniques in raspberries to control Spotted Wing Drosophila. In 2016 the St. Paul campus team observed that poly covered high tunnels with fine mesh insect netting installed on the ends reduced raspberry fruit infestation by SWD to less than 5% on average, while open plots experienced 60% infestation, even when sprayed with insecticides. We plan to investigate the use of fine mesh netting to physically exclude SWD.  This netting will be placed over a mini high tunnel frame and over the top of raspberry rows. These mini tunnels will be placed over randomized raspberry rows at different times during the growing season to physically exclude the SWD. We recognize that exclusion techniques may not offer 100% control and may not be a suitable option for all markets, such as pick-your-own operations. Despite any potential drawbacks strategies are needed to allow growers options for control of this devastating invasive fruit fly.  Finally, best practices will also benefit the growing number of MN consumers looking to increase local fruit in their diet.