Japanese beetles (Popollia japonica) are members of the scarab beetle family, which also includes June beetles and rose chafers. They begin feeding on our plants by eating small holes in the upper or lower leaf surfaces. The holes gradually widen to leave only the leaf veins, forming a lacy skeleton. Leaves may also turn brown and fall off.
Adult beetles attack more than 250 varieties of plants. Their favorites include roses, peach, quince, apple, cherry and raspberries. They tend to avoid evergreens, but shade trees such as birch, linden and horse chestnuts are targets. Preferred flowers include zinnias, marigolds and hollyhocks.
Japanese beetles can fly five miles in search of their favorite host plants. They fly during the warmest part of the day, usually after noon. They generally travel in groups, congregating around flower buds or young fruit.
Their larvae, white grubs, feed on grass and sod roots. In severe infestations, there could be as many as 50 Japanese beetles per square foot of soil. Visible damage consists of dead or balding patches of lawn that have no roots — if you can roll back the brown grass like a carpet, most likely grubs are the cause. They are fat, brown-headed, grayish-white and about 3/4 inch in length— larger than an actual Japanese beetle. They usually curl into a “C” shape.
Adult beetles are 1/2 inch long with shiny metallic-green heads and copper-colored wings. Newly emerged adults mate and the females burrow two to four inches into the soil, depositing a few eggs. They then crawl back out, feed for several days and return to the soil to deposit a few more eggs. Each female could ultimately deposit about 50 eggs in this manner, while the males continue to eat steadily!
Grubs hatch about two weeks after the eggs are laid. They feed on grass roots near the surface until fall and then burrow eight inches into the soil to spend the winter. In spring, usually April and May, they return closer to the surface, where they feed on grass roots for a few weeks, pupate and emerge as adult beetles from mid-June to mid-July. The grubs feed until September to mid-October. Injury to plant roots is most common at this time. By the end of October, grubs are mostly full grown and ready to return to deeper soil for winter.
Japanese beetle traps use a sex attractant. They are very efficient and can lure beetles from 500 feet away. Unfortunately, many of the beetles attracted do not enter the trap and will start eating your plants. If you do use a trap, place it as far away, downwind, from your plants as possible. Empty the trap often, as decaying beetles repel live ones.
Milky spore disease, caused by the bacterium Bacillus popillae, can be applied to lawns. It paralyzes the Japanese beetle grubs but can take several years to become established in the soil. Milky spore disease will not kill other scarab beetle grubs and will not kill adult Japanese beetles. To really be effective, the disease spores should be applied throughout your community as a group effort. Effectiveness is 10 to 20 years.
There are several non-organic products to control Japanese beetle grubs. Season Long Grub Control is applied once per season, anytime from May through mid-August. The active ingredient is imidacloprid, which is a systemic insecticide. It needs to be watered thoroughly after applying. Once watered in, a protective zone is formed in the soil that kills the grubs. Season Long Grub Control also contains 6-0-1 fertilizer to promote new growth in stressed lawns.
Complete Lawn Insect Control also contains imidicloprid and can be applied May through July. It also needs to be watered in but does not contain fertilizer.
If all else fails, 24 Hour Grub Killer Plus can be applied. It kills grubs by contact, delivering overnight results. It contains Dylox (Trichlorfon). It can be used when grubs are present and damage first appears. The best time is August and September, when grubs are feeding near the surface of the soil. It also must be watered in within 24 hours of application or it loses 100 percent of its effectiveness.