They like to feed in sunny areas, usually starting at the top of a plant and working their way down. Although the beetles are destructive, you can take action to prevent infestation. Treatments are available to control either the grubs or the beetle.
Removing the Beetles
If you only see a small or moderate number of Japanese beetles, experts suggest you handpick or knock them into soapy water. This is more effective if done in the evening because Japanese beetles are active feeders at night.
Starting in mid-June most grubs are in the pupa stage and insecticides are not effective. In early July adults emerge to lay eggs. The best time to apply insecticides for grubs is from July to early September.
The University of Minnesota Extension offers advice on the best way to reduce the number of grubs or insects and on the best types of insecticides to use. It’s important to use the appropriate types of treatments to control the beetles. Some pesticides are harmful to bees, so pesticides must be applied precisely.
There are articles with more information on the University of Minnesota Extension website
and on the Department of Entomology