Chisago County Master Gardener
At the Almelund Threshing Show last week, the Master Gardeners answered hundreds of questions. When we heard a question about Japanese beetles, we were able to guess the garden’s location.
The Minnesota Department of Agriculture started trapping the beetle in 1968. Between 1969 and 1979, only three beetles were captured. In 2001, over one million were trapped. They were found in 15 counties, but 99 percent were in Hennepin and Washington Counties.
At the threshing show, we heard about Japanese beetles in Marine on St. Croix and Forest Lake, so they are branching outside the metro area.
Gardeners said the beetles were eating everything in sight.
Adult Japanese beetles are oval, up to a half-inch long, metallic green with bronze wing covers. A very evident row of white hair brushes is present along each side.
The larva is a C-shaped translucent creamy white grub.
The beetles 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, feeding on foliage. They mate and return to the lawn area near sunset. Large groups often feed together.
Females lay eggs in small masses in the soil, two to four inches deep. Most eggs are laid by early August, but some are laid in September.
Adults feed on the foliage of more than 300 species. The list includes 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.
The best control method is physical removal. Pick or brush them off into a pail of soapy water.
Low-impact chemical products can be used. Neem (Azadirachta indica) repels Japanese beetles, keeping them from feeding on plants. Pyrethrins containing PBO (Piperonyl butoxide) are also effective. Both products need to be reapplied frequently.
The residual insecticides imidacloprid and inotefuran can be used to treat Japanese beetles. They are systemic, easy to apply and long-lasting. Instead of killing the beetles quickly, they cause them to stop feeding, with death coming later. These products, however, are very toxic to bees.
For more information visit www.extension.umn.edu/distribution/horticulture/DG7664.html.