Lawn Treatment for Japanese Beetles


The Japanese beetle (Popillia japonica Newman) is about 1/3 to ½ inch (8 to 11 mm) long. It has iridescent copper-colored elytra (wing casings), a dark green thorax and head and about five white setae (sets of bristles) on each side that appear as white spots. It is an introduced species, and a member of the scarab beetle family. The adults are a destructive pest of many plant species, especially roses and grapes, and of turf, landscape and food plants.

Lawn Damage

The Japanese beetle is a widespread pest of turf grass. The 2002 Annual Review of Entomology estimated that it costs the turf and ornamental industries about $450 million a year in management alone. Damage is visible as large, irregularly shaped areas of dead grass. Birds may seek out the white grubs and tear up the dead grass, raising clumps of soil.

Life Cycle

Effective control of Japanese beetles is made more effective by an understanding of the insect's life cycle. The adult males emerge from the soil first, followed by the adult females, which emit powerful sex pheromones. After mating, the females burrow 2 to 4 inches into the ground, lay one to three eggs, then emerge to mate again. This process may be repeated up to 16 times, resulting in the laying of 40 to 60 eggs.

Grubs Start to Feed

The eggs hatch in 10 to 14 days. The larvae (white grubs) feed on the roots of turf grass and other plants. This feeding is what causes grass to die back. The larvae pass through two or three stages of growth before winter temperatures slow them. In the spring, as temperatures rise, they begin to feed again--causing yet more damage to grass and other plants--before pupating near the surface of the soil. They emerge as adults in May or June, mate, and the cycle begins again.

Japanese Beetle Traps

These traps have a large backboard on which is mounted a sex pheromone attractant. A catch bag is hung below the backboard. Japanese beetles are poor fliers, so they fly at the sex attractant, hit the backboard,and fall into the catch bag. The great problem with these traps is that they attract more adult male beetles than they catch. They can be effective against small populations that have not yet become established, but otherwise cause more problems than they cure.

Milky Disease

A naturally occurring bacterial disease that kills Japanese beetle grubs, milky disease was discovered in New Jersey soil. A commercial product that contains the bacterium, Milky Spore powder, can be applied at the rate of one teaspoonful every four feet, in a checkerboard pattern. It is best applied in the fall, when the maximum number of white grubs are feeding in the soil. Infected grubs become miniature bacteria factories--when they die, billions of bacteria are released from their bodies. While Milky Spore is not an instant fix--it can take up to three years of annual applications to achieve the full effect--it then remains effective for up to 20 years.

Other Insectides

Since people and pets like to play on grass lawns, it is not a good idea to use harsh and dangerous chemical insecticides to control Japanese beetles. One safe insecticide, which is effective against the adult beetles, is neem oil. This is commercially available as an oil for dilution with water, or as a ready-to-use product under several trade names. If you decrease the number of adult beetles, it will slowly reduce the size of the infestation.

Keywords: white grubs, lawn grubs, milky spore, neem oil, beetle traps

About this Author

Peter Garnham has been a garden writer since 1989. Garnham is a Master Gardener and a Contributing Editor for "Horticulture" magazine. He speaks at conferences on vegetable, herb, and fruit growing, soil science, grafting, propagation, seeds, and composting. Garnham runs a 42-acre community farm on Long Island, NY.