Japanese Beetles

Japanese Beetle

The Insect

These foreign invaders found mostly in the eastern United States are one of your garden’s worst enemies. They’re known to infest and destroy over 300 species of ornamental plants. The major infestations occur from Maine heading south to North Carolina, and as far west as Minnesota, Illinois, Kentucky and Tennessee.

Japanese beetles were introduced in North America accidentally in 1916 on iris roots imported from Japan. They have been a growing problem because the insects entered the U.S. without their natural enemies and found both a favorable climate and an abundant food supply.

The body of the Japanese beetle is bright metallic green. The adult beetle infests and destroys over 300 species of plants including flowers, vegetables and fruits in addition to a wide variety of ornamental trees and shrubs. The larval stage of the Japanese beetle wipes out lawns, feeding on grass roots.

Life cycle

The Japanese beetle's life cycle begins in early summer when females emerge from overwintering in the ground. The female emits a scent that draws male beetles upwind to locate and mate. The female then lays eggs in lawns and grassy areas.

Dependent upon warm soil and abundant water, larvae hatch in soil and pass through several stages throughout the summer months. As temperatures cool, the larvae dig below the frost line to overwinter and continue their destructive life cycle.

Japanese Beetle Lifecycle

Weather conditions for Japanese beetle activity

Japanese beetles emerge at temperatures above 70 degrees Fahrenheit -- but preferring much warmer temperatures of 85-95 degrees. They are most active on warm, sunny days between 9 a.m. and 3 p.m.

Humidity of 60% or higher retards their flying and induces heavy feeding and destruction. They will not fly on cool, windy, cloudy or rainy days. Japanese beetles are responsive to light changes, seeking shelter when clouds pass over.

Infestation of adult Japanese beetles begins mid-June in southern states, and around July 4 in the Northeast and Upper Midwest.

Feeding characteristics

Japanese beetles prefer infected or damaged fruit and plants to healthy ones. They also prefer plants exposed to direct sunlight. They feed on the upper surface of the foliage of ornamental plants, consuming soft tissues between the veins and leaving a lace-like skeleton. Injured leaves eventually turn brown and die.

Japanese beetles begin eating low-growing plants before moving to fruit and shade trees. Later in the season, they return to attack the lower growing plants in bloom. Favorite plants include (in order of season growth) evening primrose, rhubarb, roses, grapes, sweet cherry, sassafras, apple, cherry, plum, elm, horse chestnut, poplar, willow, hollyhock, flowering shrubs, corn, asparagus, soybeans, clover, alfalfa, poison ivy, ragweed and other weeds.