Master Gardener: Japanese beetles a major problem in some areas

At the Almelund Threshing Show last week we answered over hundreds of questions. It got so that every time there was a question about the Japanese beetles, we were able to predict that the gardener was from the Twin Cities or the suburbs.

Every one of them told how the Japanese beetle was eating everything in sight. One of the gardeners at the threshing show was from Marine on St. Croix, and she reported that the beetle was in her garden. There also was a report from Forest Lake that some Japanese beetles are in a garden, so they are branching outside the metro area.

Jeffrey Hahn, assistant Extension entomologist, reports that the Minnesota Department of Agriculture started trapping the beetle in 1968. Despite traps being set up in the Twin Cities area, between 1969-1979, only three beetles were captured. In 2001, over one million were trapped in 15 counties and 99 percent of them in Hennepin and Washington counties.

The adult beetles are generally metallic green with bronze wing covers. A very evident row of white hair brushes is present along each side. The overall form is oval, and the length is up to one-half inch long. The larvae is a C-shaped white grub that is a translucent creamy white.

They spend the winter in the soil as nearly full-grown grubs that move below the frost line. As the soil warms the grubs feed on grass roots and pupate below the surface. Adults emerge in late June and early summer, feeding on foliage, when they mate and return to lawn areas near sunset time. You will often find large groups feeding together. Females lay eggs in small masses in the soil 2 to 4 inches deep. Most eggs are laid by early August, but some are laid in September. They have a one- to two-year life cycle, and according to Hahn, if you saw them this year, they probably will be around next year also.

Adults feed on foliage of more than 300 species including rose, mountain ash, willow, linden, elm, grape, Virginia creeper, bean, Japanese and Norway maples, birch, pin oak, horse chestnut, rose of Sharon, sycamore, ornamental apple, plum and cherry trees. Larvae develop on roots of various grasses.

According to Hahn, the best method is physical removal. A good way to do this is to take a pail of soapy water and brush them off or pick them off by hand so they end up in the pail. The soapy water kills them so they must be picked and thrown into the water. There are some low impact products that can be considered. Products containing Neem are effective when the numbers are low to moderate. Neem acts as an antifeedant to defer the beetles from feeding on plants. Pyrethrins containing PBO (Piperonyl butoxide) is also effective. Both products need to be reapplied fairly frequently.

There are some residual insecticides that can be used to treat the Japanese beetles (JB). Neonicotinoid insecticides especially imidacloprid and inotefuran (safari) are good choices. They are systemic and easy to apply, and they are long lasting. They do not kill JB quickly, but they do cause them to stop feeding with death coming later. One important drawback of these products is they are very toxic to bees, which is a critical consideration.

Most of the gardeners who had a JB problem had traps. They are attracted by the pheromone in these traps and while they were catching hundreds per day, the evidence suggest that this does not reduce future numbers of Japanese beetle, and it even draws more in than are caught.

For more information on the Japanese beetle and control methods, visit

–¬†Jerry Vitalis is a Chisago County Master Gardener

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