Spotted Wing Drosophila (SWD) Management Recommendations for Minnesota Berry Growers - 2019
Bill Hutchison, Eric Burkness & Suzanne Wold-Burkness
Dept. of Entomology, MN Extension IPM Program, University of Minnesota, St. Paul, Minn.
Spotted wing drosophila (SWD) is a serious pest of nearly all small fruit crops throughout Minnesota. SWD was first detected in Minnesota in August 2012 with confirmation of flies throughout the state by October of that year. Since 2012, first trap catches at most locations in the state have usually occured in mid to late-June, with rapid population growth from July to September. As populations grow, and susceptible fruit crops begin to ripen, growers need to make management decisions to minimize the risk of infestation. Growers should begin taking management actions to minimize SWD effects on their crops at first detection. Several steps can aid in reducing infestations that include shortening harvest intervals to decrease the exposure of susceptible fruit to egg laying, cooling fruit as soon as possible after harvest to minimize egg hatch or larval development, if berries have been infested, the use of exclusion netting (e.g., row covers, high tunnels) if feasible, and finally, chemical controls. The use of insecticides has been the primary management option used for SWD as other effective options have been limited thus far.
SWD has become one of the most damaging, invasive pest species we have dealt with in Minnesota. For those new to SWD, this vinegar fly species is very unique. Although SWD is similar in size and appearance to more common fruit fly species, rather than only being attracted to over ripe or fermenting fruit (as per your “standard fruit fly”), SWD are attracted to healthy, intact fruit. SWD females have a unique, serrated ovipositor (designed to lay eggs) that can puncture the skin of many berry species, lay 3 or more eggs per berry, and thus produce numerous larvae (white maggots) per berry.
Fruit are susceptible to SWD infestation from first coloring through harvest, and SWD will lay eggs in over- ripe fruit as well, making sanitation a key for this pest. This period of susceptibility is where management action should be focused. If SWD flies are present in traps, or larvae are found, the available management options and best strategies will depend on the level of infestation, management approach (e.g. organic or conventional), and the timing relative to harvest date. Currently, there is no economic threshold that’s been established for SWD, so a conservative approach is to use fly capture on your farm, or regional SWD catch data, as an indication of early activity, and to initiate protective measures if berries are at a susceptible stage. If fruit are ripening or ripe and SWD flies are trapped, growers should: 1) Implement cultural controls where possible, and 2) Protect fruit until harvest using registered insecticides.
SWD on the surface of a black raspberry.
Cultural controls can help reduce reproduction and survival of flies and should be included in the overall plan for SWD management. Cultural controls include scheduling timely harvests and removing over-ripe fruit from fields and then disposing of them properly, to minimize host plant resources for SWD. In small fields, this may be done by hand, but that is impractical in large farms. Removal of alternate wild hosts that produce fruit which SWD can infest such as buckthorn, honeysuckle, or wild raspberries (black caps), from surrounding wooded field edges or windbreaks can be helpful in reducing population increase.
If infested berries are found either in the field or after packing, there are some strategies for killing SWD before they complete development and emerge to continue infesting fields. Recent research in Oregon has shown that bagging fruit inside clear or black plastic bags will contain adult flies, and placing these bags in the sun will kill SWD. Conversely, freezing berries is another way to kill SWD larvae in fruit.
Properly applied, insecticides can be effective, but insecticide applications must also be coupled with timely harvests in between sprays, and close adherence to the re-entry and pre-harvest intervals for each insecticide must be observed (see Table below). Given the potential for rapid population increase by SWD, active management is needed until the end of harvest. For updated insecticide control options, please refer to the annual revised “Midwest Fruit Pest Management Guide”.
This guide is produced by UMN Extension and cooperating Midwest state Extension Specialists. To focus on the primary SWD information, see pp. 88 (index for other crops). Remember to always follow the pesticide label for any pesticide applications that are made whether conventional or organic products are used.
Organic fruit growers should be aware that the insecticidal control options available to them are, in general, less effective than conventional insecticides against SWD, and will require more timely application. However, experience in the west coast states and in Michigan indicate that SWD can be controlled in organic production through more intensive monitoring, timely applications if flies are detected, and shorter intervals between sprays. It is also extremely important to implement cultural controls to help reduce the overall population level. This includes pruning to open the canopy and improve spray coverage, and making sure berries are harvested on a timely schedule. A recent 2016 publication on organic options from Michigan State is also available.
The potential for insecticide resistance to develop is a concern with SWD because of the rapid development of each generation of flies and the general tendency for Drosophila flies to develop insecticide resistance. In addition, with the relatively high levels of insecticides being used to manage this pest, the increased exposure of SWD flies can lead to a more rapid development of insecticide resistance. One of the most effective approaches for reducing the likelihood of resistance to insecticides is to rotate among chemical classes. This can be done by using the chemical class as a factor when choosing which products to use. Conventional growers should be rotating among organophosphate (Malathion), pyrethroid (Brigade, Mustang Maxx or Malathion), and spinosyn (Entrust) or spinetoram (Delegate) active ingredient classes as they spray throughout the season. Organic growers are limited to using Entrust, Pyganic, and Grandevo WDG. Grandevo is a new bacterial based product that has shown excellent efficacy against SWD in recent Michigan State trials. The current challenge is getting adequate product for distribution sites in Minnesota.
Some insecticide labels will have specific information about rotating chemical classes to avoid resistance development. For example after two applications of a spinosyn-containing insecticide such as Entrust or Delegate, growers must rotate to a different class of insecticide. Resistance management is more challenging for organic growers who have few registered products that are effective against SWD.
Research updates, and new control options from Minnesota and surrounding states, will be posted to the FruitEdge page when available.