Japanese beetles have been present in the United States for over a century, affecting orchards, vineyards, and gardens across much of the eastern and midwestern states. The insects feed on more than 300 plants, including fruit trees and grapevines, and can cause defoliation that impacts yield and fruit quality.
In Wisconsin, vineyard managers report varying levels of pressure. Adam Rabe of Dirt Road Vineyards noted that Japanese beetles have been minimal in the southern part of the state in recent years, though growers in the driftless region continue to spray. He said, "The threshold for treatment for some growers may be 5-10 beetles on a vine, but I'd say 30% defoliation on a leaf is about the average threshold."
According to Christelle Guédot, fruit crop entomologist at the University of Wisconsin–Madison, the beetles have expanded their range northward in the last decade, with detections as far as Bayfield, Spooner, and Door County. Populations fluctuate with weather conditions, particularly spring moisture levels.
Management strategies vary. Commercial vineyards often apply insecticides on an as-needed basis, with spray windows timed to limit impact on beneficial insects. Other methods include row covers, though costs are high, and pheromone traps, which may attract additional beetles but can be used to concentrate spraying. Biological controls such as nematodes targeting larvae and natural predators, including birds and parasitic flies, are also used. Ground cover management may reduce egg-laying sites.
Beetles show a preference for certain fruit cultivars. Grower Bill Zeleske of Fondy Fruits & More reported damage to table grapes and Honeycrisp apples, noting that wasps also damage grapes, blackberries, apples, and pears on his farm.
In addition to beetle pressure, disease is a challenge. Rabe observed increased downy mildew following rainfall, while earlier in the season, black rot was an issue. He emphasized the importance of scouting vineyards regularly to prevent crop loss.
A new threat is approaching Wisconsin. The spotted wing drosophila, already present in northern Illinois and Michigan, lays eggs in healthy ripening fruit. Larvae feed inside, leading to softening and rot. The University of Michigan reports that the pest can cause severe losses for grape growers. Rabe stated, "Growing fruit in Wisconsin and the Midwest is very challenging. It's really having boots on the ground and eyes open and having the resources like folks in the Wisconsin Winery Association to help."
Source: Wisconsin State Farmer