Four speakers address Spotted Wing Drosophila (SWD) at the International Strawberry Symposium, Quebec, 2016

Pam Fisher, Berry Crop Specialist, OMAFRA

Spotted wing drosophila (SWD ) is a new pest worldwide, causing unprecedented losses to berry crops in North and South America, Europe and Asia. Researchers from 4 different regions reported on this pest at the 2016 International Strawberry Symposium.

Justin Renkema, Gulf Coast Research and Education Center, in Florida, is looking at factors affecting survival of immature stages of SWD in strawberry fields. Typically, SWD larvae emerge from fruit, drop or crawl to the soil and pupate in the upper few mm of soil. Dr. Renkema found that sandy soils with normal (10%) or high levels (15%) of soil moisture favoured the successful pupation of larvae, however, when soils were very dry (0% soil moisture) very few SWD survived to form the next generation. In hot dry sandy soils of Florida, there was no benefit to removing unmarketable fruit from between the rows - the number of SWD in marketable fruit was the same regardless of field sanitation.

Vaughn Walton, Oregon State University, discussed the environments where SWD is found and showed it is highly adaptable to harsh climates. The availability of early nectar sources, such as honeydew on conifer trees, may increase survival rates of this pest in the landscape. Dr. Walton demonstrated that open pruning of blueberry bushes would reduce SWD populations substantially due to differences in humidity and temperature in the canopy of open pruned vs poorly pruned bushes. Another way to make the microclimate less favourable for SWD is the use of weed mat or landscape fabric as a ground cover. This forms a barrier to the soil where SWD pupate and reduces the survival of SWD due to lower humidity and higher temperatures at the soil surface. Dr. Walton also shared some long term studies on parasitic wasps in Korea that could be potentially used for biocontrol of SWD in North America.

Catherine Baroffio, Agroscope IPS in Switzerland, discussed SWD management in Switzerland where there are only two active ingredients registered for SWD control in berry crops. Growers use sanitation, mass trapping, and exclusion netting to supplement SWD control in some crops. For sanitation, growers destroy infested fruit in a huge sealed vat where it is left to ferment before disposal. Raspberry growers improved control by shortening harvest intervals from 4 days to 2 days. Mass trapping, using an attractant developed in Switzerland, is promising for berry crops, but not for tree fruit. However, mass trapping requires very high numbers of traps throughout the season, focussing first on winter trapping in the woods, then adding traps every 2 m around the crop perimeter just as fruit begins to turn colour. Dr. Baroffio is also interested on the potential for food grade lime applied to fruit to discourage SWD from laying eggs. A great deal of information on Dr. Baroffio's research program can be found at : www.drosophilasuzukii.agroscope.ch. Growers typically spend 5,000-15,000 euros per ha on SWD control!

Miet Boonen, PC Fruit, in Belgium, reported on behalf of a large team of researchers , about the toxic and repellent effects of erythritol on SWD in the lab. Erythritol, a sugar alcohol and food additive, had a repellent effect on egg-laying SWD females, and was toxic to adult flies and larval stages when ingested. Although these initial lab studies are promising, field studies will be needed to clarify the conditions where this might be useful for SWD management.

Summary - researchers around the world are working to find ways to manage SWD in berry crops. In the future, low risk repellents, combined with mass trapping and natural enemies will be used in an SWD management program. Most researchers agree however, that a combination of strategies will be necessary for SWD control - don't expect a silver bullet.