Recent rains have caused berry splitting for grape growers around Minnesota, leading to spotted wing Drosophila infestations. Insecticide applications are warranted to prevent damaging affects on wine quality, and must be done at least one day prior to harvest.
With the wine grape harvest upon us, and a recent increase in Spotted Wing Drosophilla (SWD) counts, via our weekly trapping network, growers should be aware of the potential contamination of juice or wine, if berries are exposed to SWD just prior to harvest. The concern with late-season SWD in grapes is two-fold, including a) the potential for direct contamination of fruit/juice via egg-lay and larval infestations, and b) the surface exposure of adults and/or larvae contributing to increased levels of volatile acidity (VA) at either the juice stage or during fermentation --- which is responsible for a vinegar taint to the wine.
Regarding the risk of direct berry infestation, our 2017 research, via a graduate student (Dominique Ebbenga), we found that with intact (non-damaged) berries, the vast majority of MN grown varieties were resistant to egg-lay. Among 34 varieties tested, only 2 commercial varieties (Swenson red, Vanessa) experienced egg-lay and larval infections. However, the complication this year with recent heavy rainfall, is that excess splitting is underway with several varieties, and splitting allows SWD to easily lay eggs and generate larval infestations.
Regarding concerns with VA, Ebbenga's research found statistically significant, and higher levels of VA in Frontenac, Marquette and La Crescent juice, following exposure to 20 SWD adults (vs zero adults) for 10-14 days prior to harvest. For more information on VA, and how this contributes to a vinegar taint, see the enclosed fact sheet from Penn State. This study is being repeated this year, and will be extended to assess VA levels in wine as well.
In brief, in preparation for this year's harvest, if growers are in an area where trap catches, or adult numbers are high, or have their own traps to confirm, and one or more varieties are splitting, an insecticide application prior to harvest is warranted. Unfortunately, there are few options available with a short pre-harvest interval (PHI). Mustang Maxx is the primary pyrethroid product to consider, with a 1-day PHI and 12 hr re-entry interval; this is a Restricted Use Pesticide (RUP), and the label should be reviewed. In other words, growers must wait at least one day after applying Mustang Maxx before harvesting.
For recent SWD counts at selected TC-metro locations, see: https://www.fruitedge.umn.edu/swdtrap