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Spotted wing drosophila – Lessons from the trenches in B.C.


November 30, 1999
By Tamara Leigh

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The 2010 growing season was the first full-season experience with the spotted wing drosophila (SWD) in British Columbia.

The 2010 growing season was the first full-season experience with the spotted wing drosophila (SWD) in British Columbia. The flies were first detected in B.C. in September 2009, late enough in the season that there was no significant fruit damage. Going into 2010, no one knew what to expect. As the season progressed, the number of flies exploded, causing damage to mid- and late-season crops.

“A few growers and packers learned the hard way that this pest is a real, serious threat from which fruit needs to be protected,” says Mark Sweeney, berry specialist with the B.C. Ministry of Agriculture. “They are taking this much more seriously this year.”

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The 2010 season ended with trap counts more than 100 times higher than the season before. What is more, adult flies have been trapped through the winter, despite several significant cold spells.

“Pest pressure could well be heavier,” says Sweeney. “Numbers may build to damaging levels earlier in the season and present a greater threat to early maturing crops like June strawberries and early raspberries and blueberries.

“Growers need to be prepared for this onslaught and should be developing a management strategy,” he adds.

There are four components to a SWD management plan: monitoring, controlling alternate hosts, maintaining good sanitation practices around harvest, and using a strong spraying program for pesticides. Control will be most effective if all growers within an area participate.

Field monitoring starts in early June in the Okanagan, Fraser Valley and other fruit-growing areas and have continued. While monitoring is useful to improve the understanding of the insect’s life history, it will also indicate when to start a spray control program.

Research from E.S. Cropconsult Inc. suggests that monitoring in hedgerows surrounding fields is as important as trapping in the fields, particularly as populations start to build up in the early season. They recommend trapping as soon as the crop starts to cover, and monitoring traps weekly.

In 2010, flies were first detected in early June, and counts remained low until mid-July, exploding in mid-August. This meant that early crops escaped without significant treatment or damage, but later varieties were prone to infestation.
“Until other effective control strategies are developed, insecticide sprays will be the key method of control for SWD,” says Sweeney. “Sprays are applied when flies are detected and when ripe fruit is first present.”

In 2010, emergency registrations were obtained for Ripcord, Malathion, Delegate and Entrust (for organic fields). SWD can quickly develop resistance to pesticides, making it important to rotate through the available control products.
A preharvest clean-up spray reduces the risk of early damage, followed by additional applications every seven to 14 days through the harvest period to prevent flies from moving in from neighbouring areas. Post-harvest treatment is also advised in fields adjacent to later crops to prevent the unharvested fruit and ground fall from becoming SWD breeding sites.

Adult flies are killed by direct spray and exposure to spray residues on leaves and fruit. It is important to get spray penetration through the canopy, but required water volumes should be less than what would be required for control of other foliar pests and diseases. 

Strawberries and raspberries are relatively easy to spray with conventional equipment. However, mature blueberries represent a major challenge, as the heavy canopy makes it very difficult to access the field without causing significant damage and fruit drop.

“Helicopter spraying is being used in the U.S., but coverage is a concern and aerial will not likely be an option in the Fraser Valley,” says Sweeney. “For ground spraying, better pruning and training is required to keep fruiting area upright to allow improved field access.”

Organic growers will need to be particularly careful with SWD management. There is only one approved, registered and effective pesticide that is suitable for organics, and many producers prefer not to spray at all.

“We are very concerned with impact on organic growers,” says Sweeney. “In the Fraser Valley, we now have a significant amount of large-scale and small-scale organic blueberries. It’s going to make organic growing even more difficult than it already is.”

Managing alternative hosts and maintaining good field sanitation are important in managing SWD populations. The flies have an amazingly wide range of host plants, many of them commonly found in hedgerows along field edges. Early fruiting plants such as Indian plum, salmon berry and seedling cherries are a concern. Their early fruit provides a site where the over-wintering generation may feed and breed before crops begin to ripen.

Late-fruiting berries such as Himalayan and evergreen blackberries are a concern because they are very invasive and widespread, providing food and breeding sites from July through November. Blackberries are also believed to be a key over-wintering site. Controlling these alternate hosts in areas immediately next to fields through mowing or herbicide treatments in the fall will help keep SWD numbers down.

The timely harvest of ripe fruit, and shortening of the harvest intervals, will help reduce the likelihood of SWD infestation. Adult flies are particularly attracted to overripe and rotten fruit. California raspberry growers have found the careful removal and disposal of cull fruit has reduced pest pressure. This is easier for hand harvesting operations than for mechanical harvesters.

The response to SWD in British Columbia has been a close collaboration between provincial extension staff, crop consultants, federal research staff and growers. Emergency registrations are being sought for pesticides, and trapping and monitoring will continue for years to come. Consistent surveillance and proactive management will be critical in minimizing the damage to crops, and the economic losses to growers.

In light of the continued monitoring and research in B.C., a half-day of information sessions on SWD has been scheduled during the Pacific Agriculture Show, Jan. 27, 2012. The sessions are being held during the afternoon in the TerraLink Room and will cover everything from SWD biology and behaviour to SWD sprayer options in blueberries.

Spotted wing drosophila in Ontario
According to recent reports from the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), spotted wing drosophila (SWD) is trying its best to establish itself in the province.

In 2011, the pest was found at 21 agriculture sites across Ontario, representing almost half of the 50 monitoring sites established in 16 counties across the province. Counties where SWD were trapped during 2011 included Essex, Kent, Middlesex, Oxford, Norfolk, Niagara, Halton, York, Durham, and Prince Edward. Crops it was found in included peaches, apricots, raspberries, strawberries, blueberries, grapes and blackberries.

During 2011, researchers with OMAFRA were able to collect fruit from unsprayed areas and rear SWD flies from it.

According to the ministry’s SWD report, this indicates the pest can infest fruit in Ontario, but there have been no reports of problems or infested fruit on a commercial scale.

During the 2012 Ontario Fruit & Vegetable Convention, being held Feb. 22 and 23 in Niagara Falls, Ont., SWD will be on the agenda. Dr. Rufus Isaacs of Michigan State University will discuss the biology and management of spotted wing drosophila in berry crops during the afternoon Berry Session on Feb. 22.