Spotted Wing Drosophila (SWD) Flights Continue to Increase: Mid-July Update

July 11, 2018

Bill Hutchison, Eric Burkness, Izzy Bur, Dominique Ebbenga & Suzanne Wold-Burkness
MN Extension IPM Program, Dept. of Entomology, University of Minnesota, St. Paul campus

SWD adults, at most trap locations, reflected substantial population growth as of July 3rd. It’s clear that SWD did not take any time off for July 4th and has continued its population growth this week. With three key locations reporting last week, reflecting a range of fruit crops and fruit maturity, we are updating selected locations here. Via our Scentry trap network, for the period June 27th to July 3rd, for fall raspberries (all still vegetative stage), average trap catch increased to 30.3/trap/wk at Rosemount, UMORE Park (Dakota Co.). SWD trap catch at Waverly (Wright Co.) increased to 13.2/trap/week in summer raspberry (mostly in high tunnels; up from 1.40/trap the previous week); here, approx. 95% of the berries are beginning to change color, with 5% red. At Forest Lake (Washington Co.), trap catch in summer raspberries increased to 55.7/trap/wk, up from 3.33/trap the previous week, and 33/trap/wk in blueberries, up from 1/trap the previous week; here, summer raspberry and blueberry had about 75-80% of the fruit changing color. SWD is known to attack early maturing fruit as it begins color change, especially for raspberry. Although the final trap catch numbers for July 10th are not yet available, most of the traps at both Forest Lake and Waverly, were catching between 75-200 SWD adults/week (July 4-10th), near summer raspberry and/or blueberry. Trap catch was also high at the wood-line at Waverly, ranging from 68-360/trap, as if July 10th.  

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).

SWD on trap

Scentry Trap for Monitoring SWD Adults near black raspberry, southeastern MN (2017); note small SWD adults on outside of trap, waiting to enter via small 1/8” size circular holes. Recent research (Michigan State) has also shown the red color is attractive to SWD. S. Wold-Burkness.

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, the trap catch was again 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.

Time of Day Studies: In 2017, 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.

As nearly every small fruit grower knows, SWD is one of the most damaging, invasive pest species we have dealt with in recent memory. For a given berry crop, and when SWD and susceptible fruit are both present, control is necessary/essential for marketable produce. For those new to SWD, this fruit fly species 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 per berry. Given our experience the past two years, and given the increase in trap catch this week, if susceptible fruit (of any of the berry crops) are present, the first insecticide sprays 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. For more details, view the SWD management articles on our UMN FruitEdge web page: https://www.fruitedge.umn.edu/swd

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SWD on black raspberry