Native to Japan, the Japanese beetle was first introduced into the United States in 1916. The Japanese beetle occurs in all states east of the Mississippi River, with sporadic infestations reported in California, Iowa, Missouri, and Nebraska.
The host range of this insect is very large, over 300 species of plants. Adult Japanese beetles feed on and damage some field crops, ornamental plants, shrubs, and garden plants.
The larvae feed on roots of many turf grasses, field crops, ornamental plants, and vegetables including Kentucky bluegrass, ryegrass, bentgrass, corn, soybeans, tomatoes, and strawberries.
This insect is generally considered an infrequent pest of soybeans due to the defoliation it causes. Japanese beetle adults also may feed on silks of corn plants reducing pollination and kernel set.
Description:
Japanese beetle adults, approximately ½ inch in length, are metallic green with bronze-colored elytra (wing covers). Just below the elytra, on each side of the abdomen are six tufts of white hair. Both the sexes of the adult beetles have the same markings, but the males are usually smaller in size than the females.
Eggs of the Japanese beetle vary in color from translucent to creamy white. Under high magnification, tiny hexagonal punctures appear on the surface of the egg. Newly laid eggs are about 1/20 to 1/16 inch in diameter and ellipsoidal. As the eggs mature, they double in size, and take a more spherical shape.
The larvae, or grubs, are about 1 inch in length. The C-shaped grubs are creamy white with a brown head capsule. The immatures possess chewing mouthparts and have three pair of legs on the thorax. The grubs are similar in appearance to other commonly found white grubs in the soil.
Japanese beetle grubs can be identified by the V-shaped pattern of bristles on the raster. Pupae of Japanese beetles resemble the adult, except the legs, antennae, and wings are closely folded into the body. As the pupal stage nears its end, the color of the pupa changes from a pale tan color to metallic green.
Life Cycle:
Japanese beetles have a univoltine life cycle (one generation per year). They overwinter as third in star larvae in the soil below the frost line. As soil temperatures warm in the spring, the larvae move closer to the soil surface. They complete pupation and emergence begins in late May and early June.
Peak emergence generally occurs 4 to 5 weeks after initial emergence. At that time, emerging beetles crawl or fly in search of host plants to feed and mate. Japanese beetles are naturally gregarious and tend to aggregate on host plants.
In mid-June, female beetles begin laying eggs in the soil. During the day, females will leave host plants, burrow into the soil, and lay between 1 and 4 eggs at a time. Mating and egg laying occurs throughout the entire life of the adult beetle which lasts from 30 to 60 days. a female may lay between 40 and 60 eggs during her lifetime.
Depending on temperature, eggs hatch in about 10 to14 days after they are laid. Larvae complete three in stars, while feeding on plant roots and decaying plant material.
Injury:
Japanese beetle adults feed on more than 300 plant varieties. These insects exhibit a preference for some plants over others, thus damage to preferred plants is more severe. The beetles defoliate host plants, chewing tissue between the leaf veins, leaving a lace-like skeleton.
They generally feed on foliage on the upper and outermost parts of the plant, working their way down the plant. Severely injured leaves turn brown and drop off the plant. In corn, Japanese beetles cause injury both as grubs and adults. As grubs, they feed on root hairs, interfering with the uptake of water and nutrients.
Injury caused by grubs will often go unnoticed until plant growth and development is affected. Nutrient deficiencies such as phosphorous may cause the purpling of corn stems. Heavily infested fields may have significantly reduced plant stands and may result in replanting. Japanese beetles feed on corn silks as adults, interfering with the pollination process.
Extensive silk clipping may lead to incomplete ear fill and yield loss. In soybeans, grubs also feed on the root hairs of seedling plants, though the injury is generally not of economic importance. The main threat of Japanese beetle adults in soybeans is the defoliation of soybean plants.
Much of the defoliation of soybeans occurs during the reproductive stages of plant development. Feeding on flowers may interfere with pollination. Japanese beetles are part of a complex of insects that feed on soybeans.
Individually, Japanese beetles may not cause sufficient injury to warrant treatment, but when combined with the feeding of other soybean defoliators, treatment may be needed occasionally.
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