While several weeks later than last year, Japanese beetles definitely have arrived in Minnesota. I saw several on a garden tour last weekend and gardeners on the MSHS Facebook page chimed in that many have been seeing the beetles for at least a week. Jeff Hahn, one of the bug experts at the University of Minnesota Extension Service, wrote about them on July 10.
Here’s the 411 on Japanese beetles.
Food, Appearance, Lifecycle
Japanese beetles are most likely to begin their attack with roses, but once they get in a yard or garden, they will eat almost anything available, including geraniums, hollyhocks, Linden trees, beans, grapes, vines and fruit trees. Beyond roses, raspberries are a particular favorite. They typically feed in groups and it’s easy to spot them at the top of plants because of their iridescent green color and relatively large size. While not harmful to humans or animals, a bad infestation of Japanese beetles can defoliate a plant in a day.
Beetles are about a half-inch long with a shiny, two-tone green and tan back. They are distinguished from other beetles, such as the rose chafer, by their tufts of white hair, five on the side and two at the rear. The beetles overwinter as grubs in the soil and may be a more serious threat to turf grass than to plants, but they can fly long distances, so the beetles in your yard did not necessarily hatch there. Adult beetles emerge in July and for the next six to eight weeks go on a feeding frenzy. In the two months she is alive, a typical female Japanese beetle will lay about 60 eggs.
If an infestation is not severe, the best approach is to hand-pick the beetles and drop them in a bucket of soapy water, where they will die. You will tend to see more beetles on sunny days. It’s also a good idea start picking and killing beetles when you first see them because the presence of beetles attracts other beetles. Do not squish the beetles, no matter how tempting! It only attracts more.
You can find beetle traps for sale, but their effectiveness is not proven. Because the traps are baited with the scent of geraniums and roses as well as the pheromone of beetles, some researchers believe the traps actually attract more beetles to your yard.
Insecticides can be used on the beetle grubs (in May) and on the adults. However, many popular insecticides, such as those using carbaryl, bifenthrin, and permethrin as an active ingredient, also can be highly toxic to birds, bees and fish. The infestation will not last forever. Whenever you spray, follow package directions carefully and be mindful of possible spread of the insecticide.
—Mary Lahr Schier