The Japanese Beetle—Not Our
Favorite Import From Japan

The Japanese beetle is a highly destructive plant pest that made its way to North America from Japan. It was first found in the 1930s in a nursery in southern New Jersey.

The beetle had no natural enemies here, and found a favorable climate and an abundant food supply. This combination has made the Japanese beetle a serious threat to American agriculture.

The adults and grubs (the larval stage) feed on the foliage and fruits of several hundred species of fruit trees, ornamental trees, shrubs, vines and vegetable crops.

The adults leave behind skeletonized leaves and large holes in leaves.

The grubs develop in the soil, feeding on the roots of various plants and grasses, and often destroying turf in lawns, parks, golf courses, and pastures.

The adult is a little less than 1/2 inch long and has a shiny, metallic-green body and bronze-colored outer wings.

Their Life Cycle

During the feeding period, females intermittently leave plants, burrow about 3 inches into the ground, usually into turf, and lay a few eggs. This cycle is repeated until the female lays 40 to 60 eggs.

Since the grubs are buried in the ground about 4-8 inches deep in late fall and early winter, they stay underground until spring rolls around and they can start their damaging journey.

Then the grubs return to the turf and begin to feed again on roots until late spring, when they change into pupae. In about 2 weeks, the pupae become adult Japanese beetles and emerge from the ground. This life cycle takes a year.

Controlling Japanese Beetles

One of the main enemies of these beetles are ground moles, which loves the grubs. Unfortunately, they ruin any yard as they dig their way to those grubs.

Insecticides are effective in controlling these beetles, but they must be used carefully due to the serious hazards to people, wildlife, and the environment. Scientists believe that biological control agents are preferable to pesticides.

Biological control agents, listed below, last longer in the environment. More importantly, they do not adversely affect non-target or potentially beneficial organisms.

Beneficial nematodes, insect-eating nematodes, and microscopic parasitic roundworms seek out grubs in the soil. The nematodes have a relationship with a single species of bacteria. After penetrating a grub, the nematode inoculates the grub with the bacteria. These bacteria reproduce quickly, feeding on the grub's tissue.

The nematode then feeds on these bacteria and continues through its own life cycle, reproducing and ultimately killing the grub. These nematodes are available online and through garden shops.

Bacillus thuringiensis (Bt) is a naturally occurring soil bacterium typically used as a microbial insecticide that affects only the grub, not the adult beetle. This bacterium is a stomach poison that the grub must eat for it to be effective.

Milky spore is the common name for the bacterium Bacillus popillae. These spores germinate in the grub's gut, infect the gut cells, and enter the blood, where they multiply. Milky spore disease can suppress the development of large beetle populations. But it works best when applied in community-wide treatment.

Parasites are a proven method to reduce pest populations. There are two parasites of the Japanese beetle that have been brought to the United States. These parasites are now functioning as important biological control agents of the beetle.

Tiphia vernalis is a small parasitic wasp of the grubs. It resembles a large, black, winged ant. It's believed to live throughout the northeastern United States and south to North Carolina. It stings a grub and puts an egg on the grub. When it hatches, the larva eats the grub.

Istocheta aldrichi is an internal parasite of the adult Japanese beetle. The eggs are usually laid on the thorax of the female beetles. When they hatch, the maggots bore into the beetle's body cavity, killing the beetle.

Properly nourished, healthy plants and trees are better able to deter these beetles.

People can deter the population of pest insects by making the habitat less suitable for them, including plant resistant plant species and use mechanical traps to attract and trap the adult beetles.

When planting, make more extensive use of trees, shrubs, and other plants that these beetles don't prefer. Select plants that are least likely to be seriously injured.

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