Tracking isotopes to illuminate Nature’s grand recycling program

If you noticed higher prices or fewer raspberries at your local farmer’s market this fall, you’re not alone. Wisconsin raspberries faced a new pest, Spotted Wing Drosophila (Drosophila suzukii) this year and the big question is: where is it now and will it return?

Native to Southeast Asia, Spotted Wing Drosophila (SWD) is a fruit fly with a sclerotized ovipositor capable of piercing the skin and successfully laying eggs in ripening fruits. The fly was first described in Japan in 1916 and detected in Hawaii in 1980. By the mid-2000s, SWD was in central California & spread rapidly throughout the Pacific Northwest and Florida. In 2010, SWD was in the Southeastern US, Michigan, and Wisconsin, but no significant crop damage was reported in Wisconsin until August 2012 when it was detected by raspberry growers in 17 counties.

While other fruit flies rely on fermenting or damaged fruit, SWD’s ability to attack ripening fruits can cause total crop loss, making it a potential pest for raspberry, blueberry, blackberry, grapes, strawberries and cherries in Wisconsin. Other fruit varieties at risk (e.g. elderberry, aronia) have been gaining popularity, especially in the Driftless area where growers are trending towards higher value perennial crops to increase sustainability and income. Wineries, farm stands, and value-added products are major tourism draws to the Driftless area, as well as other fruit growing regions in Wisconsin such as the southern shore of Lake Superior and Door County, and these customers tend to have a zero to low tolerance for insect larvae in fruit, so minimizing infestations are critical.

SWD are generalists which means they can jump from host-to-host throughout the year depending on ripening fruit availability. As many Wisconsin vineyards and small fruit operations are part of diversified farms, they offer the perfect season-long availability of food.  Together, these characteristics may create ideal conditions for high local population growth and significant crop damage in the Upper Midwest. Current recommendations are limited to culling fruit and heavy season-long pesticide sprays, which simply are not options for the many growers who use organic, IPM or no-spray practices.

How did Spotted Wing Drosophila reach Wisconsin? The two main theories are 1) summer winds blow the flies in from southern locales 2) local overwintering. Field monitoring and laboratory tests have confirmed that SWD can overwinter in California and the Columbia River Gorge in Oregon. While Wisconsin winters are usually much harsher than the Pacific Northwest’s, the ability of small protected populations to survive (in thicker-skinned wild fruits, leaf litter, soil, heated buildings) may be enough to cause or supplement annual outbreaks.

To help answer the overwintering question, the new fruit crop entomologist, Christelle Guedot and I  have constructed 21 apple cider vinegar traps at 5 locations in Dane County with confirmed infestations at small fruit farms, natural areas, and raspberry high-tunnels. We will continue monitoring traps throughout the winter months to detect any flying adult presence which indicates overwintering in Southern Wisconsin. If adults are overwintering, they may fly on warm, sunny winter days, so although our traps are few, they may be one of the only attractive ‘fruits’ available in the dead of winter. Our first month of trapping (mid-December thru mid-January) only trapped two male SWD in a fall-infested high-tunnel; high-tunnels may be especially at risk for infestation due to their controlled mild temperatures and weather protection. Our second month (mid-Jan thru mid-Feb) trapped no SWD.  Starting in April, in a partnership with DATCP, we will be coordinating a farmer-based monitoring program at 15+ farms statewide to track population trends.

So will we see SWD this year? We suspect a combination of benign overwintering conditions, 2012 infestation status and landscape effects will determine if any particular county or raspberry patch will face infestation. Monitoring this year will show if SWD is overwintering and help growers make early management decisions.