No vacation for Spotted Wing Drosophila (SWD): Substantial population increase at most locations
Bill Hutchison, Eric Burkness, Izzy Bur, & 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. With three locations reporting so far, 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 31/trap/wk in blueberry (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.
Of interest, at Waverly, trap catch was again much higher at the wood-line, near the 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. In 2017, one of us (Suzanne Wold-Burkness) conducted a Time of Day (TOD) study in southeastern MN, and confirmed that SWD trap catch increased each day between 6-10am, and from 6-10pm; between 10am and 6pm, very few SWD were trapped, suggesting limited fly movement. From a management view, this also supports the recommendation for growers to spray for SWD during the evening hours (6pm to 10pm), or if necessary, early in the morning. However, the evening time period is much preferred, so as to minimize insecticide exposure to bees and other pollinators.
Again, all traps are checked on Tuesdays this year, with the SWD Trap page usually updated by Wednesday afternoons. To stay updated on the SWD situation, plan to view the following SWD Trap Network page.
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.