Japanese beetle impacts on a Minnesota cold-hardy wine grape, Frontenac: Fruit quality and harvest delays

Japanese beetle (JB), (Popillia japonica), is a highly polyphagous, invasive species, first recorded in Minnesota in the late 1960’s. Historically, research on this pest in the Midwest U.S. focused primarily on ornamental and turf crops, with little effort focused on adult feeding and subsequent damage to fruit crops. Wine grape producers have observed a substantial increase in defoliation due to JB, which has led to increased insecticide use and concerns regarding how this may impact yield or juice quality. To address these concerns, studies were conducted during 2020-2021 to determine the degree to which JB feeding has any impact on wine grape quality and yield. Trials were established using the wine grape, Frontenac. Whole vines were caged within 80-gram mesh netting and infested with JB densities of 0, 25, 50, and 100 beetles per meter-row; an open plot consisting of natural infestations was included. Each treatment was replicated four times. Beetles used for infestation were collected from natural populations of JB; adults were allowed to feed until grapes were ready for harvest. During harvest, data collection included leaf samples for average percent defoliation, cluster weights, Brix, pH, and titratable acidity (TA). Results from these studies demonstrate that as the number of beetles and defoliation per meter increase, there is a significant decrease in Brix values at harvest (P< 0.05). This delay may affect the timely harvest of wine grapes, and subsequently lower the quality of marketable yields prior to first frost in northern temperate regions.