Fly pest can gain the upper hand without vigilance

02/06/2014 02:01:00 PM
Renee Stern

spotted wing drosophila trapHannah Burrack, North Carolina State UniversityYou can make an inexpensive spotted wing drosophila trap using a drink cup with a lid and bait of sugar and yeast. For more information, visit North Carolina State University.Buyers' zero tolerance for spotted wing drosophila is pushing Southeastern blueberry growers to extra vigilance against the prolific and nimble pest.

With no natural enemies currently available to keep the fly in check, growers have few tools other than frequent pesticide applications. Spotted wing drosophila reproduces rapidly; each female can lay up to 600 eggs and a single season can see up to 16 generations.

"It's a multiplying fool," says Joe Cornelius Jr., chairman of the Georgia Blueberry Commission and president of J&B Blueberry Farms in Manor. "It can get real bad in a hurry."

The development of pesticide resistance is "a major fear," Cornelius says.

The region's blueberry crop loss in 2012 topped $23 million.

Cornelius' 2012 crop was devastated after extensive rain—20 days out of 22—upended his spray program. "We left a lot of fruit in the field in 2012," he says. Last year was also rainy but broken up by a few hot, dry days that kept spraying on track.

"It's a risky proposition to have even a potential infestation," says Hannah Burrack, associate professor of entomology at North Carolina State University in Raleigh. "They go from zero to 60. You may have a few larvae one week and the next week they're everywhere."

 

A preventive approach

That speed underlies recommendations to take a preventive approach. Ash Sial, assistant professor of entomology at the University of Georgia in Athens, suggests growers begin pesticide sprays when berries start turning color from green.

But Oscar Liburd, professor of fruit and vegetable entomology at the University of Florida in Gainesville, urges using baited traps to gauge pesticide timing.

"Spraying every seven to 10 days isn't sustainable," Liburd says.

Spotted wing drosophila zero in on ripening fruit odors.

"In the middle of the season, the power of fruit volatiles from the field overpowers (trap lures)," he says.

"But in the beginning of the season, before the fly population gets really high, you can use traps effectively. With good monitoring I believe you can reduce the number

of sprays."

As more berries ripen, growers should also sample fruit as an extra check, Burrack says.

 

Traps don’t always tell the story

Trapping has proved frustrating for Cornelius. In 2011, high counts in traps didn't translate to significant numbers of the pest in his fields. The next year, when his traps caught hardly any, he lost more than 200,000 pounds of blueberries.

"We need a bait that attracts flies before they get to the fruit," Sial says. Ripe fruit emits more complex odors that will always beat out current trap baits.

A pheromone attractant would be the ideal lure, but isn't yet available, he says.

Trap design and color appear to make little difference in their effectiveness. It's all about the bait, Liburd says.

Yeast-based baits tend to work best, but must be renewed at least weekly. At seven days, decomposing yeast attracts a wider range of insects, making it harder to identify spotted wing drosophila. And after 10 days, he says, the pest begins to reproduce in the bait.

Apple cider vinegar is the next-best bait, he says.

Liburd recommends lidded plastic cups as traps. With wire hangers and pinholes that allow fly access, they cost less than $1 each.

 

Treatment tradeoffs

The most effective pesticide controls now are pyrethroids, spinosyns and organophosphates. Rotating among the different classes from week to week is critical for resistance management, the researchers say.

"No one tool will last the entire picking season," Burrack says. Growers must take into account environmental conditions, preharvest intervals and residue tolerances.

That often means balancing tradeoffs. Mustang Max [zeta-cypermethrin] has a one-day preharvest interval but isn't accepted by some export customers. Malathion has a short preharvest interval and fewer export issues, but isn't as effective under rainy conditions. And Delegate [spinetoram] offers excellent results with longer residual effects—but isn't available to organic growers.

Entrust (spinosad), one of two effective options for organic growers, is limited to six applications per season, Sial says.

Other possibilities include Hero [zeta-cypermethrin and bifenthrin] and, still awaiting registration, both cyazypyr T and sulfoxaflor, Liburd says.

Given spotted wing drosophila's wide host range and strong flying abilities, growers should keep watch outside their field borders. Liburd says early-season perimeter sprays may provide more value, but that theory requires further study.

Attacking nearby wild hosts—especially wild blackberries—will reduce overall populations, he says, suggesting a 10-foot buffer.

 

An organic approach

Donna Miller, owner of the certified organic D&J Blueberry Farms in Inverness, Fla., relies on both Entrust and PyGanic [pyrethrin], especially for border areas in fields surrounded by woods. She monitors traps daily, most placed in field perimeters to guard against incoming flies.

"You have to scout every day," she says.

Picked fruit goes directly to an on-site inspection table, where any maggot finds can be linked quickly to a specific location that's then stripped for disposal.

Miller says the ripening patterns for some varieties put them at greater risk.

Spring High ripens early. To have enough harvested fruit to market, she may need to leave fruit on the bushes a little longer than the three-day picking cycle she strives for.

Farthing berries can take up to five days to gain full color on all sides. The longer hang time for both varieties provides spotted wing drosophila more opportunity to attack.

Frequent, thorough harvesting helps minimize spotted wing drosophila's threat, but weather and labor availability can interfere, Burrack says.

Removing or destroying culls reduces additional attractants from fields, Sial says.

But striving for a thoroughly clean floor may not be sustainable, Liburd says. "Flies won't go after [fallen fruit] when ripe fruit is on the plant," he says.

That's not a risk Miller is willing to take. She has workers rake whenever she spots a significant amount of fruit on the ground.

She's also switching over to weed fabric with new or renewed plantings; about half her 30-acre farm is now under ground cover as a protection against the pest pupating on site.

"We'll see this coming year if it makes a difference," she says.

The growers and researchers say they're pinning hopes on work to better understand spotted wing drosophila's biology, from finding pheromone lures to identifying natural enemies and other biological controls.

"It's not a no-win battle, but it sure is going to be a long battle," Cornelius says. "I can see this as something my grandchildren will be fighting."



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