Jeffrey Hahn, Asst. Extension Entomologist
Since the presence of spotted wing drosophila (SWD) Drosophila suzukii was first confirmed in Minneapolis at the beginning of August (see August 15, 2012 Yard and Garden News, ), there have many reports of this fruit fly in other parts of the state. As of the end of August, SWD has been confirmed in 11 counties and suspected in another three, ranging as far north and west as Alexandria (Douglas County) and down to the southeast corner of the state.
Although SWD was found for the first time this year, it is possible that it had been present in Minnesota a year or two earlier but at levels to low to be detected. Regardless of when it first appeared in Minnesota, it is likely that the abundance we experienced this year was the result of spring weather that literally carried up large numbers of SWD on storm fronts. We have seen a variety of other insects this year that were likely influenced by weather patterns blowing them up to Minnesota including insects that we would not normally see in such large numbers (e.g. variegated cutworm) and insects that normally don't occur in Minnesota all (e.g. genista broom moth)
SWD attacks many types of ripening, thin-skinned, soft fruit, especially cane fruit, like raspberries and blackberries, blueberries, strawberries, grapes, cherries, and plums. There has been some question about whether they will attack tomatoes but it is doubful tomatoes are attractive to them. If you have a garden with any of these fruits, there are not a lot of options for protecting them this late into the season. The primary control is treating the adults when they are first present to prevent them from laying eggs in the fruit. Once fruit is infested, you can not effectively treat the larvae as they are protected inside the produce. Infected fruit becomes soft and decays and sometimes becomes discolored. When you probe into the fruit, you can usually see the small white maggots that are responsible.
There are some cultural control steps you can take to help minimize SWD. First, pick the berries frequently when they are ripening. Remove and destroy any overripe or obviously infested fruit. Don't place infested produce into compost piles as they will likely be able to complete their life cycle and emerge as adults. It is better to place them in plastic bags tightly tied shut and thrown out. You could also place infested fruit in clear plastic bags and leave them in the sun; the heat should kill them if left out for 12 - 24 hours. You could also freeze them, making sure that the fruit is frozen for a long enough period to kill the maggots. Burying is not a good option as adults can still emerge even when buried down to 12 inches.
Even fruit that looks fine could be infested. Use your discretion as to whether you save or dispose of this produce. Should you inadvertently eat infested fruit, ingesting the maggots is not harmful to people. Using apparently uninfested fruit for cooking should not be a problem; any flies that are present would not survive the process.
SWD overwinters as an adult. It is unclear whether they can survive a Minnesota winter. This fly was not detected in Wisconsin in 2011 after it had been first found there in 2010. That could bode well for us but time will tell what kind of a problem we will face with SWD. It will be important to set up traps and survey for them in 2013.
If you encounter flies or maggots in fruit that you suspect are SWD, contact the Minnesota Department of Agriculture's "Arrest the Pest" hotline at 1-888-545-6684 or Arrest.the.Pest@state.mn.us. Please note the location and date of collection for the specimen.
For more information on SWD, see also the University of Minnesota's VegEdge web page.