Spotted Wing Drosophila Management Recommendations for Minnesota Berry Growers – 2016 Update
Chris Philips, Grace Sward, Eric Burkness, Bill Hutchison
Spotted wing drosophila (SWD) has become a serious pest of nearly all berry crops throughout Minnesota. SWD was first detected in Minnesota in August, 2012. Trap catches at most locations throughout the state are usually first detected in mid to late-June, with rapid population growth occurring in July to September. As populations continue to grow, and susceptible fruit crops begin to ripen, growers will need to make management decisions to minimize the risk of infestation. Growers should be responding to SWD catches in their local area by taking action to minimize its effect on crops. A first step in reducing infestations is to shorten harvest intervals (e.g., a 5-day interval is needed for most berry crops) in order to decrease the chance for this pest to develop in crops; however, this will likely not be sufficient to prevent infestation of susceptible crops, and chemical control is the primary option being used to control SWD.
As nearly every berry grower knows, SWD is one of the most damaging, invasive pest species we have dealt with in recent memory. For those new to SWD, this fruit fly species is very unique. Rather than only being attracted to ripe or fermenting fruit (as per your “standard fruit fly”), SWD females have a very unique, serrated ovipositor (designed to lay eggs) that can puncture the skin of berry species, lay 3 or more eggs per berry, and thus produce numerous white maggots (larvae) per berry.
Fruit are susceptible to SWD infestation from first coloring through harvest. This period of susceptibility is when 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. There is no economic threshold 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 through to harvest using registered insecticides.
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.
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 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, effective insecticides can keep be successful, but insecticide sprays, for example on a 5-day spray interval, must also be coupled with timely harvests in between sprays, and close adherence to the re-entry and pre-harvest intervals for each insecticide. 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”, available at: https://ag.purdue.edu/hla/Hort/Documents/ID-465.pdf
This guide is produced by UMN Extension and cooperating Midwest state Extension Specialists. To focus on the primary SWD information, and raspberry as an example, see pp. 112-113 (index for other crops).
SWD females can start laying eggs one day after emergence. This means that if flies are active and fruit is ripe, insecticides should be applied on a 5-7 day schedule rather than the typical 2-week interval. Moreover, it is important that sprayers be calibrated to provide thorough coverage of fruit, especially in the center of the canopy where the flies like to hide in the shade. Applications that attempt to cover several rows at a time are unlikely to achieve good coverage of fruit on all the rows.
Additional factors that should be consider when selecting insecticides for SWD control include, other pests present, anticipated harvest dates, insecticide re-entry restrictions, and potential impacts on existing IPM programs. With more frequent spraying, it is important to understand the seasonal limits for each product and the minimum time between sprays. A list of insecticides registered for use in small fruit that have high activity against SWD can be found in Table 1 (below). The level of control will depend on the infestation level, proper timing and spray coverage, and product effectiveness. A complete list of insecticides labeled for use on small fruit and grapes can be found in the Midwest Small Fruit and Grape Spray Guide. (Be aware that all chemicals labeled for use may not be labeled for SWD control).
As with all uses of insecticides to control pests, the label is the law and provides the official information on appropriate uses and rates. Refer to the label and any supplemental labels for the full restrictions on use in your crop. Always follow the specific label restrictions for your crop. The most up-to-date label information, including any supplemental labels allowing expanded uses for SWD, can be found on the CDMS website.
Organic fruit growers should be aware that the insecticidal control tools available to them are 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 application if flies are detected, and shorter intervals between sprays. It is also extremely important to implement cultural control 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.
The potential for insecticide resistance to develop is a concern with SWD because of the tendency for Drosophila flies to develop resistance, and because of the relatively high levels of insecticides being used to manage this pest. 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 the organophosphate (Malathion), pyrethroid (Brigade, Mustang Maxx or Malathion), and spinosyn (Entrust or Delegate) as they spray throughout the season. Organic growers are limited to using Entrust and Pyganic.
Some insecticide labels will have specific information to direct growers to rotate. 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.