By: Annie Klodd, University of Minnesota Extension Educator
Food for thought: In viticulture, we reduce our vine canopies through hours of shoot thinning, leaf removal, and hedging. So why do many of us act so swiftly to stop Japanese beetles from eating our leaves? While Japanese beetle control is important, you can increase your bottom line by waiting to spray until it really needed.
Japanese beetles are invasive insects that feed on leaves of over 300 plant species including grapevines. They aggregate (congregate) on leaves in mid-summer by emitting pheromones that attract other beetles to the area. They “skeletonize” leaves by eating around the leaf veins.
The trick is to decide how many Japanese beetles to tolerate in the vineyard before starting to spray.
Many agricultural insect pests have an “economic threshold” – a certain number of insects that must be present in the vineyard before an insecticide application will be economically beneficial. Economic thresholds help growers make sure that the benefit of an application outweighs the cost.
But Japanese beetles are different. There is no widely accepted economic threshold for Japanese beetles yet. More research is currently underway in this arena.
Without an economic threshold, growers’ tolerance levels seem to vary widely. Many growers spray at the first sign of beetles or leaf defoliation, while others wait until beetles have congregated on the vines and skeletonizing some of the leaves.
To decide when to spray, consider the following:
Vines can tolerate some beetle feeding
Research new and old suggests that healthy grapevines can tolerate some leaf feeding before fruit or plant health are impacted.
New research from University of Minnesota measured the impact of Japanese beetle feeding on yield and fruit quality of Frontenac. The researchers found that heavy leaf defoliation (About 20-50% of leaf surface eaten) decreased fruit quality by decreasing brix and increasing titratable acidity. However, it did not impact yield, and lighter defoliation had little impact on fruit quality. From that research, they recommended an economic threshold of 25-30% defoliation or 25 beetles per meter.
A 2003 study at Michigan State University enclosed 40 Japanese beetles per grapevine and let them feed. After two weeks, they caused about 6.8% defoliation, but this did not affect vine growth that year or the next. They determined that 30% defoliation at bloom was needed to decrease vine growth.
These studies suggest that Japanese beetles do not reduce vine health or fruit quality until 25-30% of the leaf surface is eaten. 30% defoliation of every leaf is higher than we typically see in Minnesota. In 2018, UMN researchers surveyed several grape varieties and found the highest defoliation levels between 12-15%, which is below their 30% suggested threshold for spraying.
Photo: Grapevine leaves with 2%, 6%, and 10% defoliation, measured using a standard laboratory leaf area meter. Dominique Ebbenga, University of Minnesota
Vine age and health:
As Japanese beetles aggregate on plants, they can defoliate a small, newly planted vine faster than a mature vine. Similarly, they are likely to do more damage to a stressed or stunted vine than one with a full, rapidly growing canopy.
Therefore, growers should prioritize protecting newly planted vines from Japanese beetles before populations do significant damage to the leaves.
When aesthetics matter:
Even if the beetles fail to impact grapevine health, many people find them to be disturbing and unsightly. This is enough to encourage some vineyard managers, especially those at public venues like wineries, to control Japanese beetles as soon as they see them even if the damage is minimal.
Japanese Beetle Management
Once a grower decides to spray insecticide for Japanese beetles, the next question is what to spray. Several insecticides are labeled for Japanese beetles. The Midwest Fruit Pest Management Guide is one of the resources that lists and compares these options.
Many insecticides provide knockdown control, killing beetles at or shortly after contact. Several neonicotinoids have residual activity, killing beetles in the 2-3 weeks following the application.
Spraying for Japanese beetles can be time-consuming and expensive. To minimize applications, consider incorporating a product with residual control rather than relying solely on knockdown products. Consider cost as well – based on the research discussed above, this application may not make a strong difference for yield or vine health.
Examples of effective active ingredients include phosmet, carbaryl, acetamiprid, fenpropathrin, cyfluthrin, and cypermethrin. Organic options include neem oil, kaolin clay, and Bt galleriae (e.g. BeetleGone!). Please consult the Midwest Fruit Pest Management Guide, a similar publication, and the product labels for more information.
Non-insecticide control measures
Very small vineyards may be able to manually remove Japanese beetles. Manual removal, commonly used by home gardeners, involves brushing the beetles off the vines into soapy water or vacuuming them off with a handheld car vacuum. This is generally not practical for commercial vineyards.
Another potential non-chemical option is to surround the vines in insect exclusion netting, which is similar to bird netting with smaller holes to keep out insects. I am not personally aware of research testing this method in vineyards, but a current study of exclusion netting on apple trees at University of Minnesota shows promise for a variety of insect pests.
In summary – Controlling Japanese beetles will be beneficial if infestations are high, but the good news is that growers need not panic upon spotting the first beetles. Take time to assess the situation and determine an economical plan of action before filling the spray tank.
To see recent research on how Japanese beetles affect grape yield and quality, see: Impact of Adult Popullia japonica Foliar Feeding Injury on Fruit Yield and Quality of a Temperate, Cold-Hardy Wine Grape, ‘Frontenac.’ https://doi.org/10.3389/finsc.2022.887659
If only it was 30%! Like real estate, Japanese beetle infestation numbers are a matter of local population density. They overwinter as larvae in grasses, so if there is a large are of turf nearby, 90% defoliation is possible. And they don’t all show up at once. You can wait till the vines are 30% defoliated, but you will still have weeks of control needed. I think this is valuable information generated by great institutions, but for some growers, it may be a small measure of help.