Visiting the garden of a friend recently, I was just about to ask that question: Have the Japanese beetles left yet? The answer, however, was right before me: a Japanese beetle purposefully working on the blossom of a rose bush. Darn!
According to the U of Minnesota, Japanese beetles generally emerge in early July and live for six to eight weeks, chomping away on plants and laying as many eggs as they can. This year, the beetles were first spotted in late June and will likely disappear — for this year — in mid-August. While gardeners are most concerned about the damage the beetles do to roses, grapes and raspberries, they are not very discriminating eaters and have also caused problems for soybean growers. An estimated 300 plants are on the menu for Japanese beetles.
This post from 2013 offers an overview on the life cycle and feeding habits of Japanese beetles as well as some information on controls. One take-away is that the Japanese beetle traps may not be effective. They do attract beetles but not just to the trap. Beetles feed in groups (as any gardener plagued by them will testify) and they bring others to the party by emitting an odor that says, “Dinner is here!” The traps attract beetles but they often will go to other plants in the garden too.
There are several pesticides that work on the beetles and organic gardeners usually will pick the beetles off plants and drop them into soapy water. No matter how tempting, do not squeeze the bugs as you pick them — this releases the odor that attracts other beetles, too, and brings more to your garden.