Spotted Wing Drosophila Update and Recommendations
blog-yard-garden-news.extension.umn.edu/2019/07/spotted-wing-drosophila-update-and.html
Authors: Bill Hutchison, Annie Klodd, Eric Burkness, Anh Tran, Dominique Ebbenga & Suzanne Wold-Burkness
MN Extension IPM Program, and UMN Extension - Horticulture
Spotted wing drosophila (SWD) is a highly invasive fruit fly that has spread to many Minnesota counties since being first reported in 2012. This fly lays its eggs in soft fruits like raspberries, grapes, blueberries, and strawberries. A single fly quickly moves from berry to berry, infesting many in a short time span. Farmers and gardeners alike struggle to keep SWD off of their fruit, implementing a mix of physical barriers, early and frequent harvesting, pesticides, and habitat modification.
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SWD Adults on ‘Jewel’ black raspberry, southeastern MN (2017), Photo: S. Wold-Burkness.
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Where is SWD, and When Did It Start Appearing This Year?
Bill Hutchison's lab at the University of Minnesota, and the MN Dept. of Agriculture have trapping networks throughout the state to monitor SWD populations. This allows us to know the distribution of SWD across MN and when it first appears every season in each area.
Our earliest SWD caught this year was May 23rd in Hastings. However, we did not see any substantial increase in numbers until this past week, when significant numbers began to show up.
Currently, SWD is very active at the Horticultural Research Center near the Minnesota Landscape Arboretum. Some tart cherry trees there have significant SWD damage, as observed by Annie Klodd this week. It’s clear that SWD did not take time off for the 4th! Once SWD infests cherries, they take on a bitter, rotten flavor quickly, and are not good to eat. SWD can also spread brown rot, a fungal disease of tart cherries (see photo).
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| A puncture hole left by a female SWD as she laid an egg inside this tart cherry. The surrounding tissue quickly becomes soft and liquidy after the egg is laid. Photo: Annie Klodd |
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| Brown rot on tart cherries. While brown rot can spread on its own, SWD has been associated with the spread of brown rot on tart cherries. Photo: Annie Klodd. |
Fruits to Watch Out For
Because SWD infest fruit quickly, especially ripe fruit, it is important to monitor your fruit closely, pick it as soon as it is ripe, and refrigerate, freeze, or preserve it immediately. Waiting even a day too long can mean that you go out to find a hole in each berry.
Cherries, blueberries, and summer raspberries are now underway and should be watched closely. All of these fruits are highly susceptible to SWD. Most June-bearing strawberries were harvested before many SWD were active, but those growing day neutral strawberries must monitor their strawberries closely.
Spotted Wing Drosophila Traps on Your Farm
If you would like to monitor SWD in your yard or garden, you can set up an SWD trap. Dr. Hutchison's lab uses Scentry traps, but you can also make your own vinegar traps.
Managing SWD in your Garden
For those new to SWD, this fruit fly is very unique. Rather than only being attracted to ripe or fermenting fruit, SWD females have a unique, serrated ovipositor that can puncture the berries of many fruit species, lay multiple eggs per berry, and thus produce numerous larvae (maggots) per berry.
Controlling SWD is not a single-weapon approach. Effectively managing it requires an integrated plan. While no one tactic will 100% control SWD, many growers will use a variety of tactics in combination to reduce problems as much as possible, such as:
- Harvesting every day, as soon as fruit become ripe
- Refrigerating or freezing fruit immediately after picking, to prevent the larvae in the fruit from decaying the fruit further (Note: This does not work for tart cherries, as SWD egg-lay rapidly causes fruit decay and bitterness worse than with other fruits)
- Mowing and clearing space around the trees/shrubs to reduce habitat
- Laying black plastic under the trees/shrubs to reduce SWD habitat and reproduction
- More intensive pruning to reduce SWD habitat in the canopy, and to encourage earlier ripening on blueberries (Note: Habitat reduction is likely most effective on larger plantings)
- Installing exclusion netting securely over shrubs, using low tunnel or caterpillar frames
- Growing fruit in high tunnels (there have been mixed results with this method)
- Applying effective organic or synthetic pesticides: Spinosad (org), pyrethrin (org), carbaryl, and malathion
Timing Sprays and Protecting Pollinators: If using an insecticide to control SWD, whether it be an organic pesticide or conventional pesticide, spray during the evening hours (6pm to 10pm) or very early in the morning (before 8am) if necessary. Avoid spraying between 8am - 6pm. SWD activity is highest in the evenings, with very little activity during the day. Furthermore, spraying in the evening helps reduce the risk of accidentally killing pollinators which are active during the daytime. Spraying should be repeated every 5-6 days until the harvest season is over for the fruit you are growing.
Unfortunately, we do not currently have effective biological control options for SWD.
For more information on managing spotted wing drosophila, visit Spotted Wing Drosophila in the Home Garden or FruitEdge.
To stay up to date on the SWD situation, view the complete SWD Trap Network data at: https://www.fruitedge.umn.edu/swdtrap.
