Japanese Beetles

June 1, 2017

Japanese beetles 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 surfaces of leaves. The holes gradually widen to form a lacy skeleton, leaving only the leaf veins. Leaves could also turn brown and fall off.

Adult beetles attack more than 250 varieties of plants. Favorites are 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 among others.

The beetles can fly five miles in search of their favorite host plants. They fly during the warmest part of the day, usually after 12:00 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 which have no roots. You could literally roll back the brown grass with few tools. If the grass is firmly rooted and can’t be rolled back like a carpet, most likely grubs are not the cause. The grubs are fat, brown-headed, grayish-white and are about 3/4″ in length. They usually curl into a “C” shape.

Adult Japanese beetles are 1/2″ long insects with shiny metallic-green heads and copper-colored wings. Newly emerged adults mate and the females burrow 2″ to 4″ deep into the soil, depositing a few eggs. They then crawl back out, feed for several days, then 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 8″ into the soil to spend the winter in an earthen cell. In spring, they return close to the surface, usually between  April and May. 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.

Control Measures

Japanese beetle traps use a sex attractant to lure beetles. The traps 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 if possible. Also, empty the trap often as decaying beetles repel live ones.

Milky Spore Disease, caused by a pathogenic 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.

Season Long Grub Control is a product that only needs to be applied once per season, anytime from May through mid-August. The active ingredient is Imidacloprid (Merit) 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 has an added benefit in that it contains 6-0-1 fertilizer to promote new growth in stressed lawns.

Complete Lawn Insect Control is a product that also contains Imidicloprid and can be applied May through July. It also needs to be watered in thoroughly but does not contain any fertilizer.

If all else fails, “24 Hour Grub Killer Plus” can be applied. This product 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 young and feeding near the surface of the soil. It also must be watered in within 24 hours of application or it loses 100% of its effectiveness.

For questions or comments: ron@primexgardencenter.com