Jeffrey Hahn, UMN Extension EntomologistActually they are already here as their presence was reported on June 21 (in the Twin Cities area). Japanese beetles are typically first active in the Twin Cities the first week of July but the early spring allowed them to emerge sooner than normal. These beetles are broadly oval and about 3/8th inch long with a bright emerald green head and prothorax (the area directly behind the head) and shiny bronze colored wing covers. An important distinguishing feature are the five small white tufts of hair along each side of the abdomen and two larger white tufts on the tip of the abdomen.
Japanese beetle adults feed on over 300 different plants, commonly eating the foliage of rose, grape, linden, birch, crab apple, cherry, birch, Norway maple, mountain ash, and willow. They skeletonize the foliage, eating the leaf tissue between the veins. They particularly like to feed on plants in sunny areas and typically will start eating leaves at the top of plants and work their way down. The adults also commonly eat flower blossoms, like rose. Japanese beetle grubs are also pests feeding on the roots of turf grass.
Don't be tempted to use pheromone traps to control the Japanese beetles in your yard and garden. Although they can capture what appears to be an impressive amount of beetles, research has proven that these traps attract more Japanese beetles into the area than they actually capture. You are likely to see more Japanese beetles on your plants as a result. Pheromone traps are a useful monitoring tool to determine if Japanese beetles are in the area but they are not meant to control them.
If you are interested in using a low impact insecticide, try a product containing neem. This insecticide deters Japanese beetle from feeding but it's much less effective on large numbers of Japanese beetles. Spinosad, usually effective against other foliage feeding insects, does not have much effect against Japanese beetles. There are a variety of residual garden insecticides that you can spray on the leaves of plants, including carabaryl and pyrethroids such as bifenthrin, cyfluthrin, lambda-cyhalothrin, or permethrin. Repeat applications may be necessary, especially if large numbers are present.
There are a couple of systemic insecticides available to home gardeners, imidacloprid and Safari (dinotefuran). If you are treating trees and shrubs, there will be some lag time (days or weeks) while the product moves up the plant. However, these insecticides are generally long lasting and should only require one treatment during the summer. There has been concern recently about imidacloprid adversely affecting pollinating bees so it would be best to avoid treating plants that are attractive to bees.
Some people want to manage adult Japanese beetles by treating their lawn for Japanese beetle grubs. This would work if the Japanese beetles in your yard and garden only came from your property. However, Japanese beetles are quite mobile, and there will still be a lot of them that will come from outside your yard to find your garden. Only treat your lawn if you are finding damage due to Japanese grubs but don't rely on treating your grass to reduce Japanese beetle adults.