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COUNTRY LIFE IN BC • FEBRUARY 2020


Research promises help to control SWD Spotted Wing Drosophila behaviour holds key to its management


by RONDA PAYNE


ABBOTSFORD – A cool, wet summer meant spotted wing drosophila wasn’t a major issue for Fraser Valley berry growers in 2019. The fruit fly, first identified in California in


August 2008 and in BC the following year, is a constant threat. Unlike other fruit flies which attack rotting fruit, SWD attacks ripe fruit, making it unsaleable. “[They’re numbers are] lower than expected


given the cool wet summer,” says David Mutz of Berry Haven Farm Ltd. in Abbotsford. But not all areas have been so lucky. SWD was a “very, very high” threat for


Oregon cherry growers last year, according to Oregon State University entomologist Vaughn Walton. He has assembled a team of researchers at OSU to study the fly and develop better control methods so growers will be able to stay ahead of the pest. “When you’re looking at the problem, generally speaking … you usually think about the fruit that’s being attacked, so there’s been a lot of study about the fruit,” he explains. “There’s a newer train [of thought] where some scientists aren’t necessarily looking at the host plants, but looking at the insects themselves.” Walton explains that fruit produces substances that attract SWD, but evidence shows that the insects are also producing substances that work as attractants. An attractant compound recently patented by OSU mimics these substances in order to draw insects away from fruit. OSU is now in the third year of testing the attractant in the field. It’s showing


its ability to reduce damage significantly, and commercial grower tests could take place this year. However, commercial release – and availability to Canadian growers – has yet to be determined. “Growers know about it and they really want to start using it, but we’re not quite at that phase,” he says. Regardless of what substances are being used, Walton says growers need to


manage the pest. “You need to make sure you do the best you can with those cultural methods as well. There’s an example of a grower that we’re working with who had two fields,” Walton explains. “One field is overgrown, lots of vegetation, overhead irrigation and very high [pest] pressure. In this field, he often needs to apply pesticides more frequently.” In the other field, with drip irrigation and no


surrounding vegetation, pressure is lower and the control methods are far more effective. (Walton notes that overhead irrigation washes away sprays, reducing their efficacy. Drip irrigation reduces humidity in the crop canopy, making it less conducive to the pest.) Due to irrigation, pesticide spraying and


FILE PHOTO


environmental conditions, adult flies prefer to lay eggs in the centre of the canopy. Walton advises that increased canopy pruning can reduce this protective microclimate and egg- laying. The same goes for managing surrounding vegetation. “Surrounding vegetation has more optimal microclimates than the crop,” he says.


“Temperatures on weed fabric are less optimal for survival of pupating SWD larvae.”


SWD prefers (in order): raspberry, strawberry, blackberry, cherry, peach,


blueberry and grapes. The key fruit characteristics are firmness, sugar and acidity. “It’s not possible for us to say that if you’re having a 1% increase in Brix


you’re getting a 10% increase in eggs. There are cultivar differences, more ideal environmental conditions and other factors,” he explains. “Generally speaking, when you get colour, when your Brix increases, that’s when your risk increases.” Generally, fruit is too firm for SWD to lay eggs when sugar is below 10° Brix,


explains Walton, meaning the easiest way to determine fruit susceptibility is measuring sugar content. He says SWD lays 50% more eggs when sugar rises above 10° Brix.


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