This week, the scouting at the West Madison Research Station (WMARS) vineyard occurred on 7/15/19. Grape phylloxera galls continue to be observed on most cultivars with leaves starting to fold over in many cultivars, including Brianna, Frontenac, Itasca, Petite Pearl, and Crimson Pearl (Figure 1).
The new kid on the block since our last issue is the return of the infamous Japanese beetle (JB)! Japanese beetles emerge towards the end of June and we started observing them about 2 weeks ago. Interestingly, looking back at a previous article on the same date two years ago (7/17/2017), really large aggregations were observed at WMARS (Figure 3). The good news is that we are nowhere near this level of infestation or damage this year.
While there is no formal economic threshold for JB in grape, a Michigan study suggests that grape plants can tolerate damage (withstand a certain level of injury without reduction in fruit quality and vine productivity) with up to 30% leaf area loss, at which point, the plants start showing a decrease in plant productivity. Caging 40 beetles on a grape plant at veraison for two weeks led to less than 7% defoliation and this had no effect on growth parameters, such as cane diameter, cane length, numbers of nodes, pruning weights, and next year’s growth (Mercader and Isaacs, 2003). Grape plants are really vigorous and are able to tolerate a fair amount of defoliation, thus we recommend using this 30% defoliation as a threshold for spraying an insecticide to manage JB in grape. For management recommendations please refer to the article on Japanese beetle in this issue.
At the Peninsular Ag Research station in Door County there was no sign of insect damage aside from a couple leaves of Frontenac with a few grape phylloxera galls.
Thanks to Andi Nelson and Annie Deutsch for scouting at the research stations.
Happy growing season!This article was posted in Grapes and tagged Grapes, Japanese Beetles.