Lynda Ellis, Extension Master Gardener in Anoka County.
Are you singing the Japanese beetle blues? Do you remember how many there were last year? They are a serious pest of turf and ornamental plants. They fly from plant to plant, feeding on hundreds of plant varieties, including roses, hollyhocks, and linden and crabapple trees.
They damage flowers and foliage, and can eat the soft parts of leaves until just a skeleton of leaf veins remains. The skeletonized leaves are a good symptom of Japanese beetle infestation. They are not native to Minnesota, but have been here for over a decade.
Japanese beetle adults are approximately 3/8 inches long, with a dark, metallic green head and metallic dark tan wings. They have diagnostic white turfs of hair: two in the rear and five on each side. Japanese beetle larvae (grubs) are “C” shaped. Pictures of beetles and grubs, and look alike insects, are found at the link at the end of this article.
Adult Japanese beetles feed in July and August. Their average life span is 60 days. During that time, each female lays about 60 eggs in turf. The grubs hatch soon, feed on grass roots, and by September are almost full-sized. They overwinter and again feed on grass roots starting in May.
Adult Japanese beetles can travel large distances to feed, so even large numbers of adults do not mean there are grubs in the surrounding turf. Grub damage to turf causes brown areas that lift up easily because the roots are gone. Grubs from other insects can also damage grass; identification of the type of grub is important for control.
Inspect your plants frequently. If you see a Japanese beetle, flick it into a container of soapy water, squish it, or flick it to a hard surface and step on it. Some people have reported success covering rose bushes with cheesecloth.
If Japanese beetle grubs are found in large numbers in surrounding turf, beneficial nematodes or insecticide use on young grubs in mid-July to early September is best to reduce Japanese beetle numbers next year. More information on use of nematodes and insecticides for Japanese beetle grub control is found at the link at the end of this article. Always read and follow label directions if using insecticides to control Japanese beetle grubs.
If you sing the Japanese beetle blues now, perhaps next year you can learn another song. For more information, see: http://www.extension.umn.edu/garden/insects/find/japanese-beetles/