Learn how to use organic pesticides and attract natural Japanese beetle predators such as the Tiphia wasp to control the Japanese beetle population in your garden.
This article is part of our Organic Pest Control Series, which includes articles on attracting beneficial insects, controlling specific garden pests, and using organic pesticides.
Since first being found in New Jersey in 1916, Japanese beetles have become major garden pests in eastern North America. Appearing in early summer, the coppery beetles feed on leaves of roses, grapes, beans, hops, and more than 300 other plants, including lindens and several other landscape trees. Larvae feed in lawns and weaken the grass by destroying roots.
Organic Japanese beetle controls include repeated handpicking, poultry predation, row cover barriers, and beneficial nematodes or diatomaceous earth applied to lawn areas. Japanese beetle traps require special handling, because they can attract more beetles than would otherwise be present.
This summer pest feeds for only about six weeks, but its voracious appetite, copper coloring and sexual behavior immediately get gardeners’ attention. Half-inch long beetles with copper backs, green heads, and white dots along their rears are Japanese beetles. They are often first seen on roses, but also love raspberries, grapes, beans, hollyhocks, and a long list of other popular garden plants. In bad years Japanese beetles appear in swarms, typically during the second half of June and especially in areas where they are a recently established pest.
Japanese beetles emerge in early summer and begin feeding on plants they prefer. They rasp away tissues on the top sides of leaves, which often leads to jagged holes. Japanese beetle larvae, or grubs, cause lawns to feel spongy underfoot. Lawns that are badly infested with Japanese beetles show thin growth and dry out quickly. In trees, Japanese beetles often start at the top and work their way down, causing the crown of infested trees to look mottled tan.
Japanese beetles overwinter as almost-mature larvae deep in the soil, often more than 10 inches deep. As the soil warms, the larvae move upward and pupate, and emerge as hungry beetles. They feed constantly for about six weeks. Each afternoon, females visit grassy areas to lay eggs. These eggs hatch into small white grubs that feed on grass roots. When soil temperatures cool in the fall, the larvae tunnel downward 4 to 10 inches to escape exposure to winter cold. When the soil warms in spring, the cycle begins again.
Wild and domestic birds are top natural predators during late spring and early fall, the two times of the year when larvae are feeding close to the surface. Otherwise Japanese beetles enjoy rather carefree lives. In areas where tulip poplars and other host plants are present, beneficial Tiphia wasps often suppress populations. Japanese beetles naturally prosper in landscapes planted with spacious lawns and roses.
Handpicking is essential to limiting Japanese beetle damage. Early season control is particularly helpful because Japanese beetles give off chemical signals when they find good host plants, attracting more beetles. For handpicking, a couple of inches of soapy water in a pail, bowl or jug works well (use plain water if you plan to feed the collected beetles to poultry or pond fish). If you cringe at the thought of touching the beetles, tape a large funnel to the top of a milk jug or other plastic container to use as your collection device. First thing in the morning, hold your chosen container under the leaf or bough where the beetles are feeding, and brush them down into it with your hand. The beetles won’t bite you, and as long as temperatures are cool they will fall into the water rather than flying away. Within an hour, they will drown.
Many MOTHER EARTH NEWS readers report that chickens and guinea fowl have ended their problems with Japanese beetles, mostly by gathering grubs.
Provide nesting sites for robins, bluebirds and other insect-eating birds. They will seldom nab an adult beetle but will gather a lot of grubs.
Some gardeners grow trap crops of raspberries or grapes to keep Japanese beetles away from other plants. Using a trap crop with leaves at eye level makes the beetles easier to handpick.