British Columbia berry growers face very high Spotted Wing Drosophila (SWD) pressure relative to other growing regions in Canada. This pest thrives in the Fraser Valley due to mild winters, moderate summers and many alternative wild host plants, including salmonberry, Indian plum, elderberry, wild cherry, Himalayan blackberry and snowberry. Winter trapping in the Fraser Valley over the past six winters by the BC Ministry of Agriculture and E.S. Cropconsult Ltd. has shown that SWD trap catches often drop off after a freezing spell in January or February. However, we have just experienced a second consecutive mild winter, with few days below freezing, and flies continue to be caught in traps around blueberry and raspberry fields. With early spring weather, the degree day model developed by Oregon State University predicts that overwintering SWD will start to lay their eggs in the Fraser Valley in mid-May. Populations typically increase exponentially from late spring through the summer as moderate temperatures in coastal B.C. favour reproduction.
SWD is so pervasive in B.C. that raspberry, blueberry and blackberry growers can assume that their fields are at risk and need protection as soon as the fruit starts to colour. Last year, raspberry and blueberry
harvests started in mid-June. Berry growers are encouraged to use both cultural and chemical controls to manage this pest, including:
• Wet traps to monitor adult flies and inform management decisions.
• Float out tests (salt or sugar solution) to detect larvae in ripe fruit.
• Short harvest intervals. Timely harvest is critical, as late hanging fruit is highly prone to infestation.
• Regular pesticide sprays when fruit is colouring.
• Pesticide rotation to delay the development of resistance. SWD resistance to insecticides is a major concern. Multiple generations of SWD per year in B.C. increase this risk.
• Cold storage for fruit immediately after harvest to maintain fruit quality.
• Removal and management of cull fruit. Avoid situating compost piles or cull piles near berry fields.
• Management of alternative host plants. Remove flowers and fruit from alternative host plants that are growing in hedgerows adjacent to berry fields.
Different markets have different SWD tolerance thresholds. For farm direct sales, light levels of SWD infestation may be tolerable. However, for berries going to fresh-market domestic wholesale, export markets or processing, a high level of SWD management is required. Fruit that arrives at packers and processors is assessed for SWD infestation with a high level of scrutiny. For many buyers, there is zero tolerance for SWD and detection at any level results in rejection and/or downgrading to juice grade.
The BC Ministry of Agriculture continues to work collaboratively with SWD researchers at Agriculture and Agri-Food Canada’s Summerland Research and Development Centre, throughout the Pacific Northwest, and across Canada to better understand the biology and behavior of SWD in hopes of developing new tools that could minimize the impact of this pest on our berry industry.
For more information on SWD, please check out the BC Ministry of Agriculture’s Pest Alert:
Carolyn Teasdale is industry specialist, berries for the BC Ministry of Agriculture.