It’s that time of year where every trip to the garden involves an inspection for Japanese beetles. Are they here yet?
So far, in my garden in St. Paul, the answer is no, not yet. Japanese beetles typically appear in Minnesota in late June or early July. They live for 6 to 8 weeks, creating havoc the entire time, eating roses, green beans, birch trees and about 300 other plants. They mate; the grubs hide in the soil for the winter, and it’s all over before the Minnesota State Fair.
While beetles have been in the United States more than 100 years, they have only been in Minnesota about 50. In 2018, we first spotted them on June 26. In 2017, the first sighting (for me) of Japanese beetles was not until July 3. In 2015, they were here by June 24. I’ve seen none so far.
We’ve had a cool, wet spring, so perhaps they are delayed. Many gardeners are hoping that the brutal winter we had with early sub-zero temperatures and a lengthy cold period may have knocked them back, but conversations with University of Minnesota entomologists indicate that is unlikely. We did have a tough winter, but beetle grubs stay below ground—sometimes as low as 6 to 8 inches below ground.
In addition, going into the winter of 2018/2019, Japanese beetle grub numbers were high. The U has a research area in Rosemount where researchers counted 26 Japanese beetle grubs per foot. That’s a lot. Grub numbers are most affected by drought during the mating season.
Dealing with Japanese Beetles
Once they get here, beetles are a real challenge. Keep an eye out for the Japanese beetles because early interventions tend to be more successful.
Traps do catch a lot of beetles, but they also bring more into your yard. They are not recommended unless you have a very large yard and can set the trap away from your garden. The best non-insecticidal control is to knock the beetles into a bucket of soapy water. Going out in the morning is best as they tend to be out and they move more slowly then. You can also add covers to plants you want to protect. Fruit plants, however, should not be covered until blooming is finished and no pollination is required. Insecticides can work, but the effective ones tend to harm bees as well. This article from the University of Minnesota Extension covers your options and the pros and cons of each.
A long term option may be to not plant the plants JBs love the most, such as roses and grapes. It also helps to remember that this, too, shall pass, and most plants are not killed by Japanese beetles.