By: Annie Klodd, University of Minnesota Extension
In case you haven’t heard about invasive fruit fly spotted wing drosophila (SWD), this is the infamous pest that pierces the skin of soft fruits like raspberries and strawberries to lay eggs inside. It costs the US fresh berry industry millions of dollars each year.
As viticulturists whose businesses rely on high quality fruit, we may be tempted to assume that SWD will also decimate our vineyards in the same way it does berries. However, this might be a poor assumption that causes us to spend more on insecticides with no real benefit.
The question I will explore today is: How big of a problem is SWD, really, for vineyards? Is it actually any worse for grapes than other fruit flies?
For SWD to impact grapes like it does to other soft berries, it would have to be able to pierce the grapes’ skin and lay eggs inside. But grape skin may be just thick enough to deter them. Entomology researchers at University of Minnesota recently published a study that found that out of 34 different cold climate hybrid and vinifera grape varieties, SWD was only able to pierce the skin of 4 varieties.
SWD is more likely to impact vineyards by introducing sour-rot causing bacteria to split or injure grapes. However, common fruit flies already do this, and we already know how to control them. The difference may be that SWD are attracted not only to overripe berries but to ripening berries as well, stretching out our timeline for management.
The Big Fuss About Spotted Wing Drosophila
SWD is not native to the United States or Canada, but it is now prevalent throughout fruit-growing regions of North America. It was accidentally introduced from east Asia in 2008, likely via cargo as with many invasive pests. It quickly spread throughout the continent, costing the US strawberry, blueberry, cherry, and raspberry industries millions of dollars; in Minnesota raspberries alone, the pest causes over $2M per year. The costs come in the form of damaged fruit, lost marketable yield, and frequent, expensive insecticide applications.
The feature that makes SWD special from other fruit flies is that the females have a serrated “ovipositor” that they use to pierce the soft skin of ripe berries to lay eggs inside the fruit. Those eggs become larvae (maggots) that feed on the fruit, making it mushy and unsalable. Both male and female SWD can also introduce bacteria to the berries that cause fruit rots. They begin to become attracted to fruit when it is ripe or nearly-ripe fruit and do not infest green, unripe berries.
Learning what problems SWD poses for the grape industry will help growers decide if spraying for SWD is a worthwhile expense.
Researchers Explore the Impact of SWD on Wine Grapes
Entomology researchers at University of Minnesota recently found that the skin of many cold climate and vinifera grape varieties may actually be too thick for SWD to penetrate. This begs the question: Is SWD really a big deal for the grape and wine industry?
To find out if SWD can pierce grape skin, the researchers trapped male and female flies in vials with individual grapes, forcing them to mate and attempt to pierce the grapes to lay their eggs. After two weeks, they observed the grapes to see how many varieties the flies were able to infest, and how many grapes were still intact.
The grapes they used were harvested weekly between veraison and harvest, to find out whether riper berries were easier for flies to infest.
Out of 34 hybrid and vinifera varieties tested, the flies were only able to break the skin of 4 varieties: Swenson Red, Vanessa, and two non-released hybrids from the University of Minnesota breeding program. Popular cold- and cool-climate hybrids like Itasca, Marquette, Jupiter, Petite Pearl and Frontenac were unimpacted. Vinifera varieties Chardonnay, Riesling, Malbec, Valde Penas, and Pinot Noir were also unaffected by the flies.
These results suggest a few key lessons:
● Grapes are much more resilient to SWD than other berries like strawberries, raspberries, and blueberries.
● Since intact grapes are unlikely to be punctured by SWD, growers should not rush to spray for them if the grapes in the vineyard are healthy and intact.
SWD and Sour Rot Disease
The researchers’ next question was whether the flies might impact already-damaged grapes, by introducing sour-rot causing bacteria. This was likely, considering that other common fruit flies already do this.
Common fruit flies like Drosophila melanogaster carry acetobacter, the bacteria responsible for grape sour rot. They are attracted to grapes that have been split open from rain or hail, or have been pierced by birds and wasps.
When fruit flies feed on leaking, damaged grapes, the acetobacter they carry is converted to acetic acid, infecting the grapes with sour rot. When winemakers use sour rot-infected grapes in wine, the wine has an undesirable flavor and aroma.
To learn whether SWD can introduce sour rot like other common fruit flies, the researchers did a field study in the vineyard. Mesh bags were used to trap groups of SWD flies on individual grape clusters. They tested Marquette, Itasca, Frontenac, and LaCrescent, four popular cold climate hybrid cultivars from the University of Minnesota.
After about 2 weeks, they processed the grapes into wine and measured the amount of acetic acid in the wine through laboratory analysis. They found that when SWD landed on grapes with split skin, they did introduce acetic acid flaws to the wine.
This finding is consistent with other studies on sour rot, which have found that fruit flies are necessary to introduce the sour-rot causing bacteria to injured grapes.
For most cold climate grape growers, SWD may not cause any more problems than common native fruit flies already do. The biggest risk they pose for most varieties may simply be introducing infection to berries that are already injured by splitting, birds, and wasps.
Watch for SWD from veraison to harvest and consider control measures if berry injuries are observed, to reduce the risk of sour rot in the wine. This University of Minnesota study focused on cold climate hybrid varieties, testing 5 Vinifera varieties and 29 cold climate hybrids. Therefore, more research is needed to learn which Vinifera varieties are more or less susceptible to SWD infestation.
● Research from University of Minnesota found that spotted wing drosophila (SWD) are unlikely to pierce the skin or infest most grape varieties; only 4 of the 34 cold climate and Vinifera varieties were damaged.
● Like common native fruit flies, SWD are attracted to injured fruit and can introduce sour-rot causing bacteria that creates acetic acid flaws in the wine.
● Growers should monitor for grape injury from veraison to harvest, use measures like bird netting to reduce grape injury, and only if needed, apply insecticides that target fruit flies.
● Dispose of sour-rot infected grapes and do not allow them to be used in wine.
● Initial berry injury by SWD can predispose the fruit to attraction by other fruit fly species common to Midwest grapes, and thus increase overall damage and risk to juice quality.
● SWD is attracted to ripe or ripening fruit from veraison to harvest. They are not attracted to green, unripe fruit. Injuries on unripe fruit are likely due to other causes besides SWD.
Read the full study here:
Ebbenga, DN, EC Burkness, MD Clark, and WD Hutchison. 2021. Risk of Spotted-Wing Drosophila Injury and Associated Increases in Acetic Acid in Minnesota Winegrapes. American Journal of Enology and Viticulture 72(1): 106-112.
For more information on SWD, see University of Minnesota FruitEdge:https://fruitedge.umn.edu/