Images: The multicolored asian lady beetle (left; Harmonia axyridis), social wasps like yellow jackets (middle; Vespula and Dolichovespula spp.), and common fruit flies (Drosophila menogaster) are all insects that can lead to issues with grapes as harvest approaches.
Grape grown in Minnesota generally deal with fewer types of insect pests than apple production. In the past decade or so, Japanese beetles have been a top pest, mostly targeting grape leaves and devastating younger, establishing grapevines. Japanese beetle populations have been observably lower than previous years, likely due to environmental conditions and the presence of the Winsome fly (Istocheta aldrichi).
There are, however, a few other insects that can be problematic in vineyards as harvest approaches and fruits ripen, especially when issues related to bird damage or berry splitting has occurred. These include multicolored asian lady beetles, social wasps, and fruit flies.
Multicolored Asian lady beetles (MALB)
Multicolored Asian lady beetles' (MALB; Harmonia axyridis ) likeliness to becoming an actual pest will depend on a vineyard’s location in relation to where MALB adults are already feeding.
Both the juvenile and adult MALB life stages act as beneficial insects in soybean fields when they prey on aphids and other insects. However, once soybean fields have begun to dry up in preparation for harvest, MALB can move over to nearby vineyards. After they appear on site, MALB act as opportunists that feed primarily on already-damaged berries, but their presence can be impactful if they end up in the crush process, which can lead to wine taint and off-flavors.
Identification and traps for scouting
MALB look similar to other species of lady beetles, but can be identified by a black “M” shaped pattern behind their head. Yellow sticky card traps can be used for monitoring MALB in the vineyard. Place traps on the outside of a grapevine canopy near the vineyard border, especially on sides closer to neighboring fields.
Monitoring and management
Vineyards can’t always choose which neighboring crops are close by. Maintaining healthy, intact fruit, and removing clusters with berry damage can help prevent major infestations. During years with heavy rains close to harvest, however, berry splitting may be out of the sphere of one’s control as well.
For vineyards that hand harvest fruits, harvesters can shake clusters or lightly brushing off beetles. Additionally, keep harvest containers closed whenever possible to exclude MALB.
Some pesticide options can help manage MALB. Chemical management recommendations for managing MALB can be found starting on page 172 of the Midwest Fruit Pest Management Guide.
Additional reading:
Multicolored Asian lady beetle (University of Minnesota)
Social wasps
Commonly called "bees," most yellow and black striped Hymenoptera insects that cause issues around harvest are all social wasps in the family Vespidae. These include paper wasps (Polistes spp.) and yellowjackets (Vespula spp. and Dolichovespula spp.), which can be found in vineyards in the Upper Midwest, with Vespula yellowjackets especially found increasing in numbers with late-season harvested grape varieties (1).
While most social wasps are predominantly carnivores that feed on caterpillars and other insects, they still can seek out sugars for energy. Berries with pre existing injuries are more susceptible to wasp feeding, however, wasps do have the ability to damage intact berries as well.
When thinking about managing social wasps, growers should only consider killing wasps if it’s necessary due to the benefits wasps contribute as predatory insects.
Bear in mind the following considerations:
- Insecticides are not encouraged for wasp management in vineyards. There are currently no insecticide product labels that list any of the social wasp groups listed above.
- Maintaining healthy fruit is a cultural practice that prevents social wasps from becoming an issue. Harvesters should also avoid leaving dropped fruit on the vineyard floor after veraison.
- Only when absolutely necessary, manage wasps by destroying their nests and using a trap and kill approach yellowjackets.
- Identifying which wasps are problematic for a vineyard is the first step in knowing which type of nest would effectively manage specific populations. Even yellowjackets can vary in how they nest (e.g., ground-nesters and aerial-nesters).
- Trap and kill options can be effective when the appropriate bait is used at the right time (see below).
Homemade trap bait
A trap placed in Tom Plocher’s vineyard located in Washington County, Minnesota (Zone 5a). Photo taken by Tom Plocher.
Private grape breeder, Tom Plocher, shares the following formula for yellowjacket bait that can be used for traps:
2 parts water
2 parts apple juice
2 parts apple cider vinegar
½ part sugar
A few drops of dish soap added after mixing the bait
Set up traps as soon as wasps become noticeable within a vineyard. Monitor traps regularly for refilling and replacing the bait as needed.
Because wasps present a stinging issue for those who pick grapes by hand, plan to pick grapes in the morning when wasps are less active. Wear protective clothing as needed, and avoid wearing brightly colored clothing that can attract wasps.
Additional reading:
Identifying bees and wasps (University of Minnesota)
Social wasps in vineyard (University of Wisconsin-Madison)
Keeping wasps away from wine grapes (University of Minnesota)
Fruit flies
Both the common fruit fly (Drosophila menogaster) and spotted wing Drosophila (SWD; D. suzuki) can both cause issues in vineyards around harvest time. The common fruit fly is a concern for compromised berries as they feed on and lay eggs within split and damaged and can be vectors for acetic acid producing bacteria (Acetobacter spp. and Gluconobacter spp.).
Although SWD prefers to lay eggs in table grapes and other fruits that have a thinner skin (e.g., blueberries, raspberries, strawberries), they can make fruit susceptible to sour rot when they do attempt to pierce through ripening berries. The bacteria listed above, in combination with yeast populations, give way to a condition known as sour rot. Infected grapes become discolored, break down, and can shrivel up over time, all of which makes them unsuitable for wine production.
Sour rot susceptibility can also vary among cultivars, relating to cluster compactness, berry skin thickness, berry softness (2), and how early the fruit ripens.
How is sour rot different from botrytis bunch rot?
Late stage botrytis bunch rot leads to sporulation (i.e., the fuzz), which distinguishes it from sour rot; however, early stage botrytis can sometimes look similar to sour rot.
Sour rot berries can be distinguished by the scent of vinegar that the fruit gives off, and how “smooth” the berry skin remains as time goes on (i.e., no fuzz). Additionally, sour rot infected fruit will continue to attract more fruit flies due to the vinegar, yeast, and sugar combination that is present and potentially oozing at the infection site (2).
Monitoring and management
Growers who have concerns about sour rot in their vineyard should monitor for and manage fruit fly populations as a means for prevention.
Resources from the University of Wisconsin recommend creating homemade traps by using a 32 ounce deli cup with holes drilled into the lid, filled with the following recipe:
1 Tbsp Active dry yeast
4 Tbsp Table sugar
12 oz Water
2 drops of Dish soap
Traps can be installed before or after veraison depending on historical SWD populations. This trap will not work well to manage SWD populations, but is used to detect when they are on site.
Chemical management strategies can be found in the section, "Grape veraison to harvest - Insect pests" on page 172 of the Midwest Fruit Pest Management Guide.
Grape section resources cited:
Guédot, C.; Hietala-Henschell, K.; Lois, A.N. Species Composition, Abundance and Seasonal Phenology of Social Wasps (Hymenoptera: Vespidae) in Wisconsin Vineyards. Insects 2018, 9, 57. https://doi.org/10.3390/insects9020057
Hall, M. and Klodd, A. Managing sour rot and other grape bunch rots. YouTube webinar available on UMN Extension Small Farms:
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