Japanese Beetle

Japanese beetles are common pests in Northeast gardens. They were accidentally  introduced with infested irises from Japan for the 1916 World's Fair. Generally herbivores, they are known to feed on over 400 species of broad-leaved plants, although onlyabout 50 species are preferred. The grubs (larvae) also feed on a wide variety of plant roots, including ornamental trees and shrubs, garden and truck crops, and lawn grasses. They seem to especially prefer Kentucky bluegrass, perennial ryegrass, tall fescues, and bentgrass.

Description and Life Cycle

Adults are a brilliant, metallic green color, generally oval in outline, and 3/8 inch long. The wing covers are coppery brown and the abdomen has a row of five tufts of white hairs on each side. These white tufts are diagnostic. The larvae are typical white grubs. Larvae that have matured by June pupate, and adult beetles emerge from the last week of June through July. On warm sunny days the new beetles crawl onto low growing plants and warm for a while before taking flight. The first beetles out of the ground seek out suitable food plants and begin to feed as soon as possible. Females deposit eggs in soil, usually in lawns, during the summer (primarily in July). Grubs feed on roots near the soil surfaces until cold weather arrives, then move to about 15 cm (6 inches) below the surface of the soil to they hibernate for the winter. Grubs move nearer the surface in the spring and resume feeding. They pupate in May and June, emerging in early July as adults. The life cycle is completed in one year, with ten months spent as a grub in the soil and two months as an adult.

Damage

Feeding adults skeletonize leaves, leaving only midribs and other veins. When populations are high, they may defoliate plants. Roses, fruit trees, beans, tomatoes, and corn are among the favorite foods of the adult Japanese beetle. Adults feed during the day, especially in warm weather, and on plants in full sun. They chew on the flowers and the leaves, which soon wilt and drop.

Control

Cultural

• Japanese beetle quarantines are operated by the USDA-APHIS and states involved with shipping materials out of infested areas to uninfested areas. Though this has not stopped the slow progression of Japanese beetles westward it seems to haveslowed the process.

• Avoid planting trees and shrubs that are highly attractive to adult Japanese beetles near turf. These  include Japanese and Norway maple, birch, pin oak, horse chestnut, rose of Sharon, sycamore, ornamental apple, plum, cherry, rose, mountain ash, willows, lindens, elms, and Virginia creeper.

Biological and mechanical

• The bacterial milky spore disease (available as Milky Spore at local garden centers) has been effective at controlling grubs in certain areas of the eastern United States. The spore count must build up for two to three years to be very effective. However, the disease may not be performing well in certain areas. This may be due to reduced virulence, soil conditions, or grub resistance. More commonly, different white grub species have displaced the Japanese beetle grubs.

• Light infestations can be controlled by knocking the adults into a pail of soapy water early in the morning while they are still sluggish. The plants can also be sprayed with Neem, which is a botanical derivative approved for organic gardens; it acts as a feeding repellant. 

Traps: Commercially available Japanese beetle traps use two chemical lures to attract the beetles: one, a sex pheromone, attracts male beetles; the other, a blend of three chemicals which emulates a floral scent, attracts males and females. However, studies indicate that these traps do not significantly reduce grub populations and in some cases may actually contribute to increased foliar plant damage. Traps may be effective in a home yard surrounded by forest, on a golf course, or in a residential community where all properties simultaneously trap Japanese beetles.

Chemical: Chemical control of grubs in lawns is the most effective method of controlling Japanese beetle populations.

Adapted from: Marion Gold, Steven Alm, and Meggan Gould, 1999; Ohio State University Extension