Page 58 - Grapevine JanFeb 2022

Around The Vineyard

the question: Is SWD really a big deal for the grape ly, considering that other common fruit flies already
and wine industry? do this.

To find out if SWD can pierce grape skin, the Common fruit flies like Drosophila melanogas-
researchers trapped male and female flies in vials ter carry acetobacter, the bacteria responsible for
with individual grapes, forcing them to mate and grape sour rot. They are attracted to grapes that
attempt to pierce the grapes to lay their eggs. After have been split open from rain or hail, or have been
two weeks, they observed the grapes to see how pierced by birds and wasps.
many varieties the flies were able to infest, and
how many grapes were still intact. When fruit flies feed on leaking, damaged grapes,
the acetobacter they carry is converted to acetic
The researchers trapped indi- acid, infecting the grapes with sour rot. When wine-
vidual grape berries in vials with makers use sour rot-infected grapes in wine, the
male and female SWD flies to wine has an undesirable flavor and aroma.
test which, if any, varieties the
flies are capable of penetrating. To learn whether SWD can introduce sour rot
Photo: Dominique Ebbenga, like other common fruit flies, the researchers did a
UMN. field study in the vineyard. Mesh bags were used to
trap groups of SWD flies on individual grape clus-
The grapes they used were harvested weekly ters. They tested Marquette, Itasca, Frontenac, and
between veraison and harvest, to find out whether LaCrescent, four popular cold climate hybrid culti-
riper berries were easier for flies to infest. vars from the University of Minnesota.

Out of 34 hybrid and vinifera varieties tested, the Mesh bags trap SWD onto
flies were only able to break the skin of 4 variet- grape clusters, to test how
ies: Swenson Red, Vanessa, and two non-released they impact the grapes. Photo:
hybrids from the University of Minnesota breeding Dominique Ebbenga, UMN.
program. Popular cold- and cool-climate hybrids
like Itasca, Marquette, Jupiter, Petite Pearl and After about 2 weeks, they
Frontenac were unimpacted. Vinifera varieties processed the grapes into wine and measured the
Chardonnay, Riesling, Malbec, Valde Penas, and amount of acetic acid in the wine through laborato-
Pinot Noir were also unaffected by the flies. ry analysis. They found that when SWD landed on
grapes with split skin, they did introduce acetic acid
These results suggest a few key lessons: flaws to the wine.

● Grapes are much more resilient to SWD than This finding is consistent with other studies on
other berries like strawberries, raspberries, and sour rot, which have found that fruit flies are nec-
blueberries. essary to introduce the sour-rot causing bacteria to
injured grapes.
● Since intact grapes are unlikely to be punctured
by SWD, growers should not rush to spray for For most cold climate grape growers, SWD may
them if the grapes in the vineyard are healthy not cause any more problems than common native
and intact. fruit flies already do. The biggest risk they pose for
most varieties may simply be introducing infec-
SWD and Sour Rot Disease tion to berries that are already injured by splitting,
birds, and wasps.
The researchers’ next question was whether the
flies might impact already-damaged grapes, by Watch for SWD from veraison to harvest and
introducing sour-rot causing bacteria. This was like- consider control measures if berry injuries are

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