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By: Maria Smith, HCS OSU
It’s like a scene out of the 1992 classic horror movie Candyman in the vineyard this fall. Yellow jackets, hornets, bees, wasps (1; Fig. 1)… but I promise, it’s the sugar content in the grapes and not because we’re summoning a man with a hook for a hand.
Fig. 1. Bald faced hornet in Frontenac blanc, Sep 2021
So, a real question: why so many stinging friends joining us in the vineyards this fall? (By the way, it’s not just us in Ohio, this same issue has been reported this fall in Michigan, Wisconsin, and Minnesota).
One suggested answer is the transition to newer selective insecticides that, in addition to being safer for us, are safer for the wasps (2). Another is that populations of these insects are cyclical, and they may just simply be more abundant (3). Whatever the reason, they are a problem for both the human pickers and for the grapes.
As previously mentioned, stinging insects are attracted to the increasing berry sugars as the grapes ripen. They may either feed on existing damaged berries or cause damage by boring into intact berries (Fig 2). Fruit quality may then be further compromised by the vectoring of yeasts and bacteria that induce sour rot (4; Fig 3). Wasps themselves have been found to be vectors of sour rot microbes (5), while other may come in following berry damage, including fruit flies (Drosophila melanogaster), spotted wing drosophila (Drosophila suzukii), and sap beetles among others.
Fig. 2. Dessicated, damaged berries following wasp feeding, Frontenac blanc, Sep 2021
Fig. 3. Wasp damage and sour rot in Sauvignon blanc, Sep 2021
Managing Sour Rot in the Winery
With harvest wrapping up, the focus now should be on how best to manage fruit rot in the winery. A 2016 article from Penn State Extension details some key take home points on how to minimize the impacts of fruit rot in wine and are summarized here:
- Minimize rot coming into the cellar by sorting fruit from harvest
- Select an appropriate yeast strain that can survive harsher fermentation conditions
- Create a nutrient management strategy to maintain a healthy fermentation
- Monitor and manage SO2
- Improve wine sensory attributes using additives, including tannins and inactivated yeast products
- Blend with low volatile acidity (VA) wines
- Sterile filter wines prior to bottling for microbial stability
Looking to next year: ways to control wasp populations in the vineyard
Harvesting at night or very early in the morning when conditions are cooler can certainly help the pickers when it comes to exposure to wasps, but it doesn’t eliminate the problem for ripening fruit.
Control for these insects is challenging but can be more effectively managed by deploying traps along the perimeter of the vineyard at the onset of ripening (Fig. 4), eliminating nests (bee careful!), and ensuring that fruit is harvested at maturity. Additional information on trapping and eliminating nests can be found at https://ohioline.osu.edu/factsheet/HYG-2075-11.
As for insecticides, information from UW-Madison suggests that Entrust, Mustang Maxx, and Delegate have “good” efficacy when it comes to wasp control. Although these insecticides are not labeled for wasps, they are used to manage fruit flies and other pests near harvest. Keep in mind, several low-PHI insecticides are restricted use and require a pesticide applicators license to purchase and use.
Fig. 4. Purchased wasp trap deployed in Wooster, Unit 2 vineyard, Sep 2021
- Guedot, C et al. 2018. Species Composition, Abundance, and Seasonal Phenology of Social Wasps (Hymenoptera: Vespidae) in Wisconsin Vineyards. Insects. https://doi.org/10.3390/insects9020057
- Warner, G. 2014. The Increasing Problem of Yellow Jackets and Hornets. Good Fruit Grower. https://www.goodfruit.com/the-increasing-problem-of-yellow-jackets-and-hornets/
- Milkovich, M. 2020. Wasps Becoming More Worrisome for Grape Growers. Good Fruit Grower. https://www.goodfruit.com/wasps-becoming-more-worrisome-for-grape-growers/
- Ivey, M et al. 2021. Sour Rot Disorder of Grape. PLPATH-FRU-50. https://ohioline.osu.edu/factsheet/plpath-fru-50
- Madden, A et al. 2017. The Emerging contribution of social wasps to grape rot disease ecology. PeerJ. https://doi.org/10.7717/peerj.3223
Tags: 2021 Season, Wasps, Entomology, Viticulture