Spotted Wing Drosophila (SWD) Updates: Current Updates and IPM Reminders

August 23, 2017

Bill Hutchison, Eric Burkness & Suzanne Wold-Burkness
MN Extension IPM Program, & Dept. of Entomology, University of Minnesota, St. Paul, MN  55108

As we now transition into the peak Fall Raspberry season in Minnesota, it is critical that growers are well prepared with the current IPM recommendations, and strategies to be successful. This is especially true for those producing raspberries in open field situations, but also critical for those production systems in high-tunnels (HT).  Here, we offer 5 recommendations necessary to continue to be successful. It has also been encouraging to hear from growers who have had success with this system.

swd male
Figure 1. Male SWD fly with distinctive wing markings.

 SWD female
Figure 2. Female SWD fly (note the lack of wing  markings), with close-up view of the ovipositor, with serrated edge, that allows for penetrating early-stage and ripening fruit, for egg-lay in berries.

Note: Several traps are available for monitoring adult SWD (both males and females are caught in traps).

Top 5 steps for successful SWD management:

  • Be aware of changes in SWD population pressure on your farm/region, by monitoring the adult fly numbers via at least one trap/acre, or following the trap catch updates on FruitEdge, at the UMN IPM Program page (https://www.fruitedge.umn.edu/swdtrap). For example, at Rosemount (Dakota Co.), the weekly trap catch declined to >20/trap per week, and overall pressure to date, has been less than in 2016. Trap catch numbers declined this past week, which is always good news, but there is still another 6-8 weeks of the season remaining, for Fall Raspberries. 
  • Among all berry crops, raspberries are known to be the most susceptible to SWD attack, due to the thin skin of the berries, and likely sufficient attractive volatiles emanating from fruiting berries.  Based on research In other states, and work in MN, SWD preference for fruit species is summarized as:  raspberry, blackberry, strawberry, blueberry, wine grapes, cherry tomato. For example, on untreated raspberry, we have recovered up to 20 SWD flies surviving per berry, whereas we only usually see 2-3 survive, or emerge from blueberry.  This information is critical for adjusting spray timing, harvest timing, to manage SWD.  Berry preference can also be influenced by previous damage from other sources, such as diseases, feeding by other insects (ex: Japanese beetle), or bird damage.  Intact, healthy grapes (MN varieties) are not usually successfully attacked by SWD; however, if previously damaged, SWD will readily lay eggs, and larvae will survive.
  • Because of the damage potential, and regardless of the berry crop, insecticidal sprays are still essential for protecting berry crops, and timing of sprays is critical.  Spray-interval: for raspberry, we continue to recommend a 5-day spray interval be maintained, where possible (7-day interval should be the maximum allowed), as soon as the first berry color change is evident.  For each insecticide, the maximum rate/acre should be used, with high volume for max. coverage in the canopy (e.g, >25-40 gall. water/ac), and e.g., use of air-blast sprayers for coverage. Pre-harvest interval (PHI) must also be followed.  For raspberry this continues to primarily be a rotation of Mustang Maxx, Malathion and Delegate (or Entrust, for organic growers). In addition to the PHI, the re-entry interval for harvesting/worker entry must be followed. Although the 5-day interval is a challenge, this can be managed along with timely harvests (see below).  Alternating insecticides is also essential to minimize the risk of SWD developing resistance.
  • Beyond the spray-interval timing, the Time of Day to spray is also likely to have an impact. Initial, detailed time of day studies in MN this year, indicate based on trap catch, that SWD adults (males and females) were much more active in raspberry/blackberry crops, between 6 – 10pm, with very few flies caught between 10pm to 6am, or from 6am to 6pm.  Because the current insecticide approach relies on direct insecticidal contact with adults, as well as active residual activity in the canopy for 4-5 days, these results suggest that waiting until the early evening (~ 1.5 hours before or after sunset) will be most effective at controlling the adults. Also, based on honey bee, and other pollinator spp. activity, sprays in the evening should have much less of an impact on pollinators.  Of course, pollinators (esp. bumble bees) are more essential for blueberry vs. raspberry, but many growers also maintain bees for honey and other crops.
  • Clean and Timely Harvests: Timely harvests are especially critical for this pest, since SWD adults can lay multiple eggs per day, and generation time during the summer months can be as fast as 10-14 days; thus multiple pressure and need for control.  Also, berries left on the soil surface, will allow SWD eggs and larvae to continue to develop, pupate, and produce more adults to re-infest additional berries (essentially creating a refuge).  On harvest days, workers should be careful to harvest all mature berries available that day, and be prepared to harvest daily –in between spray timings –throughout the season; this is especially critical for raspberries. Once harvested, berries should be delivered to various markets the same day, or placed in refrigeration for temporary storage.
Female SWD on black raspberry

Figure 3. Female SWD fly preparing to lay egss in a black raspberry (S.J. Wold-Burkness).

Although the focus of this article has been for traditional open-field berry production, there are several High-tunnel production systems, including traditional plastic over the top, with Exclusion netting on the sides/ends that show significant promise as a successful alternative for SWD management. Our research trials have documented 98-100% reductions in SWD numbers and/or berry infestations using a HT approach. Additional results, and updated costs for this approach will be presented in additional articles this winter, in preparation for the 2018 season.