Spotted Wing Drosophila (SWD) Flights Continue to Increase: Alert and Update
Bill Hutchison, Eric Burkness, Anh Tran, Dominique Ebbenga & Suzanne Wold-Burkness
MN Extension IPM Program, Dept. of Entomology, University of Minnesota, St. Paul campus
Our earliest trap catch dates this year occurred with single flies caught SWD adults in forest locations adjacent to crops; the earliest catch was May 23rd at Hastings. We did not see any substantial increase in numbers until this past week, and especially for the period 6/28 to 7/1; both within crop fields and adjacent wooded areas. It’s clear that SWD will not be taking time off for the 4th!
Trap catch highlights: With 5 locations reporting this past week, reflecting a range of crops and fruit maturity, we observed totals of 58, 32, 5 and 2 (per 3 traps), at Houston Co., Hastings, Rosemount and Forest Lake, respectively. At Waverly, we also had a big increase to 40 flies (5 traps). For many locations, females dominated over male numbers, which we have observed before; thus infestations can develop quickly. For the forest edge areas, we saw an increase from nearly zero on 6/27 to an increase of 210 at Rosemount (30 traps total), and 54 at Forest Lake (30 traps total). As in most years, the June-bearing strawberry crop likely missed the SWD flight, with harvest complete last week. Blueberries and summer raspberry are now underway, and should be watched closely. All traps are checked on Tuesdays this year, with the SWD Trap page usually updated on Wednesday afternoon, or by Thursday morning. To stay up to date on the SWD situation, view the complete SWD Trap Network data at: https://www.fruitedge.umn.edu/swdtrap. This week, we also illustrate the Scentry Trap we now use for the network (shown below).
Trap Notes: Although our Metro Area SWD Network provides one indicator of annual pest pressure, it cannot fully reflect the SWD pressure, or onset of SWD activity, that you experience on your farm. It is therefore best to
make, or purchase two or more SWD traps for your farm, to have the best information about SWD infestation potential. We selected the Scentry trap following several years of research, and feedback from other states. The primary advantages include a) an improved 4-component lure (bait) that is more specific to SWD (and therefore fewer non-target flies to sort through), b) the lure lasts up to six weeks (less expense), and c) the catchment area is water-based (with one drop of dish detergent to break surface tension) –vs the use of apple-cider vinegar. The water-based trap therefore passes the “smell test,” making fly counts a more pleasant experience (according to my students). That said, apple-cider vinegar traps are still popular, as they can be made with home-based materials (e.g., using 1-qt or 1-gal plastic containers, etc). However, for our purposes the Scentry trap has provided consistent results. Scentry, and other SWD trap options, can be ordered from Great Lakes IPM, in Michigan, at: https://www.greatlakesipm.com/.
SWD Local Movement between Berry Crops and Forest Edge: Of interest, at Waverly, our 2018 data indicated that the trap catch was much higher at the wood-line, near a high-tunnel raspberry (vs inside the high tunnels). The wood-line (forest edge) traps were up to 67.2/trap/week, compared to the previous week at 7.33/trap. The higher trap catch in a wooded area, especially adjacent to or surrounding fruit production fields has been observed previously, including an extensive study in Wisconsin. This study, and other observations suggest there may be significant local dispersion of adults from berry fields to the wooded areas – potentially on a frequent or daily basis; e.g., SWD moving out of berry fields during the heat of the day, to the forest shade, then moving back to the berries in early morning, or late evening. Our high trap catch in the woods in 2019 is affirming of the previous results.
Time of Day Studies & Pollinator Protection: In 2017-2018, one of us (Wold-Burkness) conducted a Time of Day (TOD) study in southeastern MN, and confirmed that SWD trap catch increased considerably, each day between 6-10am, and from 6-10pm. Between 9am and 6pm, very few SWD were trapped, suggesting limited fly activity during the heat of the day. From a management view, this also supports our recommendation that growers spray for SWD during the evening hours (6pm to 10pm; first choice), or if necessary, early in the morning. The evening timing reflects the best balance to time a spray when SWD is most active in the berry crop, while minimizing the risk to pollinators (most pollinators active during the day). This practice will not completely remove the risk to pollinators, but is much preferred over sprays applied between 8am to 6pm.
IPM Tips: As every small fruit grower knows, SWD has become the most damaging, invasive pest species of fruit crops, statewide. For a given berry crop, and when SWD and susceptible fruit are both present, a management plan is essential to produce marketable fruit. 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. Given our experience the past 5 years, and the increase in trap catch this week, if susceptible fruit (of any of the berry crops) are present, the first insecticide spray should be applied as soon as possible, and maintained on a 5- to 6-day schedule until final harvest. Harvest dates must be scheduled around the spray schedule, accounting for the Pre-harvest interval (PHI) on the insecticide label for each crop, and Re-entry intervals (REI) for workers. Also, insecticide classes should be rotated to minimize the risk of resistance. Unfortunately, we do not currently have effective biological control options for SWD. The best alternatives to insecticides thus far has been the use of High-tunnels (poly on top & exclusion netting on ends/sides; or full exclusion netting). For more details, view the SWD management articles on our UMN FruitEdge web page: https://www.fruitedge.umn.edu/swd