Japanese Beetle Emergence Underway

Bill Hutchinson, Professor, Department of Entomology

Throughout much of southern Minnesota, Japanese Beetle (JB) emergence is underway. It will be several weeks before adult feeding damage becomes a concern for fruit crops, so growers have time to prepare and review pest management strategies. Adult beetles, with an appetite for >300 host plants –including several fruit crops, sweet corn, linden trees and roses, to name a few -- and their unique ability to aggregate and create a “feeding frenzy” quickly gets the attention of growers. However, most crops can tolerate high levels of defoliation. 

Photo: Whitney Cranshaw, Colorado State University, Bugwood.org

Graduate students in Entomology studied JB feeding behavior in detail the past two years.  In wine grapes, Dr. Dominique Ebbenga found that our common ‘Frontenac’ wine grape could easily tolerate 25-30% defoliation without a yield loss or quality loss. Adam Toninato, working in fall raspberries, found that ‘Heritage’ could tolerate up to 25% defoliation before observing a yield impact.

In addition, we have found that there is often a strong “edge effect” of JB feeding activity in each of these crops. Thus, insecticide sprays – if needed – can be directed to field edges vs spraying an entire field or vineyard. I will provide more information on “action thresholds” and the edge effect in future articles.

A few years ago, we developed a degree-day (DD) based, adult phenology, or timing model for beetle emergence. For most locations in southern MN, we are currently at approximately 460 DDs (59F base: 71F upper threshold), indicating early emergence (0-10%) – and we are now seeing some early beetle activity. Peak emergence is predicted to occur at 620 DDs. The model is based on the use of commercially available traps to monitor adults. You can follow the JB emergence and weekly forecasts (updated each Monday) at the automated DD page for JB at: https://vegedge.umn.edu/degree-days-midwest-insects/japanese-beetle

Fruit growers can find more information on the biology and IPM recommendations for JB adults, Spotted-wing Drosophila, and other pests at the FruitEdge site (www.fruitedge.umn.edu ). In addition, commercial growers should review the new 2023-2024 Fruit Pest Management Guide, for IPM options for all insect pests of fruit crops (Link to: https://ag.purdue.edu/department/hla/extension/sfg-sprayguide.html ). In addition to purchasing a copy ($15), growers can download the PDF version for free, then scroll through the book to view the specific crops of interest.

Finally, for those interested in an in-depth overview of the JB life cycle, identification of adults, the overwintering grub stage, and more, please review the Extension page (Link to:

https://extension.umn.edu/yard-and-garden-insects/japanese-beetles ).