Japanese Beetles

Learn how to keep these shiny beetles from skeletonizing your precious plants.

July 12, 2011

Eastern gardeners are all too familiar with how japanese beetles (Popillia japonica) skeletonize leaves and demolish plants, especially roses and grapes. The larvae are white grubs that feed on organic matter and roots of grasses in the soil; they can cause a great deal of damage to your lawn.

What to do now: Pick off or knock the beetles from plants into a bucket of soapy water. Don't bother to invest in those beetle bag traps: They tend to attract more beetles to your yard than otherwise would have been there. Parasitic nematodes (Heterorhabditis spp.) prey on the grubs, says David Shetlar, Ph.D., extension entomologist at Ohio State University. (He does not recommend milky disease spore products, which are often used to control grubs in lawns.) Apply nematodes to the soil in late August or early September, Dr. Shetlar advises, and make sure that the nematode product is fresh and the soil is kept continuously moist after it is applied.

Next year: Cover garden vegetables with floating row covers by mid-June to keep the beetles off, or spray fruit and vegetable plants with kaolin clay (sold as Surround). And let your lawn go dormant in summer, Dr. Shetlar urges. Watering to keep the lawn green, he says, "is just begging for white grub problems!"

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DESCRIPTION: Adults: chunky, metallic blue-green, 1/2-inch beetles with bronze wing covers, long legs, and fine hairs covering body. Larvae: fat, dirty white grubs with brown heads; up to 3/4 inch; found in sod. Found in all states east of the Mississippi River. Many other species produce C-shaped white larval grubs; see also White Grubs.

DAMAGE: Adults eat flowers and skeletonize leaves of a broad range of plants; plants may be completely defoliated. Adults feed on fruit, such as raspberries and plums, opening a site for disease infection. Larvae feed on roots of lawn grasses and garden plants.

LIFE CYCLE: Overwintering larvae deep in the soil move toward the surface in spring to feed on roots, pupating in early summer. Adults emerge, feed on plants, and lay eggs in late summer; eggs hatch into larvae that overwinter in soil. One generation occurs every 1 to 2 years.

CONTROL: In early morning, shake beetles from plants onto dropcloths, then drown them in soapy water; or shake beetles into a can of soapy water to drown. Cover plants with floating row cover; apply Heterorhabditis nematodes or milky spore to sod to kill larvae; attract native species of parasitic wasps and flies; organize a community-wide trapping program to reduce adult beetle population; spray plants attacked by beetles with insecticidal soap. As a last resort, spray with neem.

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