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. But other than tearing your hair out in frustration, what can you do? The tips below can help you protect your precious vegetables against these persistent insects.
Learn to Identify Japanese Beetles
Japanese beetles are found in all states east of the Mississippi River. Adults are easy to recognize: they have chunky, metallic blue-green bodies, 1/2-inch long, with bronze wing covers, long legs, and fine hairs covering body.
Larvae are fat, dirty white grubs with brown heads, up to 3/4 inch long, often found in sod. Many other species produce C-shaped white larval grubs, so make sure you've actually got Japanese Beetles and not some other helpful insect species before waging war.
The damage you're noticing can also help you identify Japanese beetles. Adults eat flowers and skeletonize leaves of a broad range of plants; plants may be completely defoliated. They also feed on fruit, such as raspberries and plums, opening a site for disease infection. Larvae feed on roots of lawn grasses and garden plants.
Understand their 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.
How to Control Them
If your problem is with adult beetles, 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. If necessary, spray plants with an insecticidal soap, and as a last resort try neem oil.
As for the grubs, parasitic nematodes (Heterorhabditis spp.) prey on them, says David Shetlar, Ph.D., extension entomologist at Ohio State University. 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. (He does not recommend milky disease spore products, which are often used to control grubs in lawns.)
If you've had problems with Japanese beetles in the past, you'll want to think ahead. Cover garden vegetables with floating row covers by mid-June to keep the beetles off, or spray fruit and vegetable plants with kaolin clay. 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!"