Damaged Grapes Most at Risk From Fruit Fly

Drosophilia can spread spoilage bacteria to cracked or damaged berries

by Peter Mitham
The female spotted wing drosophila uses its large and jagged ovipositor to break the skin of wine grapes. Photo source: Virginia Tech
Corvallis, Ore.—The tiny insect pest spotted wing drosophila has cast a large shadow over wine grape growers since 2008, when it was first identified in North America in a patch of strawberries at Watsonville, Calif.

Known scientifically as Drosophila suzukii, the fruit fly was identified among Pinot Noir vines in Oregon’s upper Willamette Valley in 2009, as well as in a non-commercial vineyard south of Abbotsford, B.C.

The fly is frightening because of a jagged ovipositor that allows it to saw into ripe fruit and deposit eggs rather than simply taking advantage of damaged fruit like the better-known species D. melanogaster. Under the right conditions, it is also able to reproduce rapidly—up to 13 generations per season in Japan—though the cooler climate of the Pacific Northwest appears to be limiting its fecundity to as little as four generations per season here.

But a round of research at vineyards across North America are giving growers some hope that the new-found pest—for all its natural equipment—may be as opportunistic as its cousin D. melanogaster with respect to wine grapes.

The relatively thick skins of wine grapes mean most varieties resist penetration by the flies’ ovipositors, reducing the potential for infestations.

While commercial wineries in Virginia found D. suzukii larvae in pressed juice last season, research by Virginia Tech entomologist Doug Pfeiffer notes that damage wasn’t uniform across vineyards and suggests that much may depend on adjacent crops and vineyard-management techniques.

For example, strawberries are a host crop, and flies may head over to ripening wine grapes once berries no longer provide what they need.

Dangers of cross-contamination

Meanwhile, one grower in Virginia lost a third of his crop to sour rot following an infestation—something research by Vaughn Walton, an associate professor at Oregon State University and entomologist attached to the Oregon Wine Research Institute, suggests may be the real risk from D. suzukii infestations.

Research at locations in Oregon and Italy suggest that adult flies carry bacteria that can promote spoilage of fruit damaged by cracking, hail injury and wildlife.

“They’re sitting on those cracked berries and they have bacteria on their mouthparts, on their ovipositors, on their feet, and those body parts get in contact with the exposed pulp on those cracked berries, and that’s what’s causing the issue,” Walton told Wines & Vines. “It’s not the larvae that are causing the issue, it’s these adults that are moving the bacteria around and increasing those spoilage bacteria.”

This isn’t to say ripe grapes aren’t appealing to the flies.

The lure of sugar content
Walton and other researchers have prepared a forthcoming paper for the journal Horticultural Entomology that indicates that oviposition (egg laying) by D. suzukii correlates with véraison and increasing grape ripeness.

“We found that oviposition increased with an increase in sugar content and a decrease of acidity levels,” an abstract of the paper states. It also says that damaged grapes are most amenable to infestation.

“Increased presence on wine grapes, as indicated by egg laying and increased longevity, was observed for flies that were exposed to incised berries as opposed to fully intact berries,” the abstract states.

But if adults take advantage of breaks in the skin of ripe grapes to deposit their eggs, larvae are a secondary issue—and curiously, Walton noted, may forestall spoilage.

The research, and particularly the potential of the flies to serve as vectors of bacteria, is renewing attention on the pest.

“Initially we were thinking, ‘They’re not developing on the berries, so it’s not going to be a problem.’ And that was the wrong question,” Walton said. “The question is, ‘Is it affecting the quality of the grapes?’ And it is.”

Research this past season, which saw record levels of spotted wing drosophila in affected vineyards, indicates that damaged grapes infested with the fruit fly have higher levels of acetic acid, which becomes a problem at wineries.

A presentation at the recent Oregon Wine Industry Symposium attracted significant interest from winemakers seeking to manage the problem.

Growers, meanwhile, wonder if it’s possible to stop the pest from entering vineyards in the first place.

Walton said he’s fielded repeated questions about buffer zones, but the most effective buffer is a 60- to 70-yard swath between vineyards and other host species—something he doesn’t consider feasible.

Biological controls are under investigation, but in the meantime growers can take precautions that may benefit themselves as well as neighboring growers.

Cherries, raspberries and blueberries are particularly susceptible to the pest, suggesting that growers in close proximity to these crops need to be vigilant not only for crossover from adjacent fields but conditions in their own fields that may allow populations to develop to economically damaging levels.

Keeping plantings clear of excess and damaged fruit is important, including removing and burying or otherwise disposing of culled fruit to limit fly infestations.

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