When faced with spotted wing drosophila (SWD), it’s critical for growers to keep in mind this is almost certainly unlike any pest they have dealt with before. That’s because it’s one of two species — out of a potential 1,500-plus — that feeds on healthy, not rotting, fruit.
That’s certainly not the end of growers’ difficulties with SWD, says Jhalendra Rijal, University of California Cooperative Extension Area IPM Advisor, Northern San Joaquin Valley. It’s tricky because vinegar flies — the tiny flies often found circling above fruit bowls on kitchen tables — are so ubiquitous. Even the oh-so-common name, “fruit flies,” indicates the pests’ omnipresence.
“You need to build an IPM program for any invasive insect that threatens one of your crops because of serious potential losses,” he says. “Once you’ve figured out what control measures might work for you — how to make a decision based on sampling — it’s time to act.”
As Rijal explained to attendees at the recent Biocontrols USA West Conference & Expo, the potential losses are large for SWD, not only because it feeds on healthy soft fruits such as cherries and berries, but also because the females lay eggs inside the fruit, where they are not exposed to insecticides.
SWD also isn’t limited in its hosts. Rijal says if none of its favorite fruits are around, the pest can survive quite easily on other fruit that has been damaged.
“SWD has a really wide host range and can feed on damaged or decayed fruit,” he says. “They have many choices.”
Not only that, but the grower or pest control adviser is constantly thinking about multiple generations, because SWD can complete its life cycle in just a week to 10 days.
If all that weren’t enough, Rijal says he’s been hearing — so far, this information is strictly anecdotal, but it is indicative — that if you apply insecticides too often with SWD it becomes resistant.
“Combine all these together and you’ve got a big problem,” he says. “You need to think about managing it, not controlling it.”
How best to manage SWD? In addition to the above, Rijal makes the following points.
- Smaller growers should consider netting for cherries and berries. “Netting is suddenly popular, and it’s great if you can put netting over your crop. I’ve seen Canadian and New York studies that showed great results,” he says. “If it’s feasible to put 80-gram netting over your whole crop, you can get great results. But with big acreage, it’s just not feasible.”
- Sanitation is absolutely critical; move the fruit that are already ripe. Take fruit that hits the ground. Remove fruit every day if possible. “With raspberry, I recommend harvesting every day,” he says, “or every two days at most.”
- Rotating insecticides is extremely important. Cherries are susceptible for four to six weeks, so you have to plan. “Spray every 7 to 10 days with different [materials],” he says. “And besides resistance, you need to consider whether you will be faced with MRLs (Maximum Residue Levels).”
- Get out there and monitor your crops. It sounds simple, but a lot of cherry growers, for example, think that when the cherries get to be straw-colored, it’s time to start spraying. But it’s much better to monitor. “Even though you might not have hit that exact threshold,” Rijal says, “you need to think about SWD and the four to five (active ingredients) you can rotate so you’re not using the same A.I.”
Rijal says there’s a lot of research being conducted on SWD all over the world. He’s confident researchers will find better answers for growers in the not too distant future, though the challenge is immense.
“We’ll see more and more with global trade reflecting people’s movement; they’ve spread so fast and into so many different places,” he says. “We may not be able eradicate them, but we can eradicate them in some areas. Ultimately the solution lies in biological control, but it is still in progress research-wise.”
Integrated Pest Management Strategy for SWD
- Monitor fields with traps and check them regularly
- Use cultural practices (harvest frequently, destroy leftover fruits, etc.) whenever possible
- Based on trap capture and the stage of fruit development (ripening or not) apply effective insecticides to protect the fruit
- Select the insecticides based on your production system (conventional vs. organic), efficacy and label requirements, and your target markets; MRL (maximum residue limits) have been a big issues for export market
- Resistance management should be a goal when planning for SWD control. Include bio-based and other softer products into rotation
- Application timing, sprayer type and efficiency, and coverage are critical