As of this week, and with the record number of days over 90F this summer, the Japanese beetle (JB) is rapidly approaching the end of its annual emergence curve. As of Monday, July 30th , we will be between 75-90% emergence for southern Minnesota. Interestingly, due to the “urban heat island” effect in the Metro area, JB development is more advanced (90% emergence) than the ambient temperatures driving degree-day accumulations in the rural areas (see DD map for July 24- 30th).
That said, JB numbers in pheromone traps will continue to remain active through August and early Sept., and some JB will be present on preferred host plants through August as well. Current JB counts, via pheromone traps, at our 3 sites this year remain in the low to moderate range.
Preferred fruit crop hosts include: Heritage fall raspberry, Honey Crisp apple and several wine grape varieties. As noted in previous years, the vast majority of feeding damage on wine grapes is on the leaves vs direct damage to developing grapes. On raspberry, JB will feed on both foliage and developing berries, shown below. For raspberry, it is important to manage JB prior to the onset of fruit production.
For both Wine grapes and Fall Raspberry, action thresholds have been established at a fairly high densities of 15-20 JB adults/3-ft row, for each crop. However, it should be noted these thresholds were developed using established plantings of each crop, where both grapes and raspberry have considerable ability to compensate for defoliation, particularly up to 25% defoliation. Of course, for wine grapes, considerable pruning of vines and leaves occurs periodically to manage vegetative growth, yet not affect final yields or grape quality. Thus, the feeding damage by JB should be considered within this context.
In addition, for grapes, with two years of research (in progress), we have found that JB usually colonize vineyard edges during most of July. This is in part due to their dispersal behavior when first locating vineyards and due to the organic volatile compounds (OVCs) common in most grape hybrids. OVCs are released by the first wave of beetle feeding, which in turn attracts more beetles to the site. Once beetles migrate to the vineyard edge, with ample leaf tissue available, they have little motivation to continue to move toward the center of the vineyard. Given the beetle’s behavior, the research has shown that for the first 2-3 weeks of July, the majority of the feeding damage is limited within an “edge effect” of the first 3-30ft of row. Thus, insecticidal sprays could be limited to the edge areas to maintain JB suppression, while also reducing insecticide use; growers may want to evaluate this option at their vineyard sites. Another key caveat for wine grapes: because the action threshold was developed for established vines, 1st or 2nd year plants, with very few leaves to sustain their development, should be monitored more closely and managed more conservatively to suppress JB feeding damage.
For more details on JB management in fruit crops, and insecticides labeled for grapes and raspberry, growers should view the current Midwest Fruit Production Guide (2023-2024), available at the Purdue web site mentioned in our JB pest profile. — By Bill Hutchison, Extension Entomologist, Eleanor Meys, Undergraduate Student, & Suzanne Wold-Burkness, Dept. of Entomology, University of Minnesota