Japanese beetle

Overview

Japanese beetles are 3/8" (8-11 mm) long and ¼" (5-7 mm) wide, brilliant metallic green insects with copper-brown wings whose hard body makes them unpalatable to many predators, including birds. To distinguish them from other metallic green or copper-colored beetles, the diagnostic sign is a row of 5 small tufts of white hairs under the wing covers on each side and a 6th pair at the tip of the abdomen. The larvae, called grubs, are grayish-white with a dark brown head. They are C-shaped when disturbed. The 1st instar (stage of an insect or arthropod between molts) is 1/16” (1.5 mm) long and the 3rd instar is 1¼” (32mm) long. They are found in the soil where they feed on the tender roots of vegetables, lawn grasses, and other plants.

Life Cycle

Japanese beetles overwinter as a partially grown grub in the soil below the frost line. The grubs resume feeding on grass roots in the spring, and then pupate near the soil surface. Adults emerge between May and July, depending on their geographic location . Adults live 30-45 days and feed, usually in groups, first on low-growing plants and then on tree leaves, working from the top of the plants downward. After feeding and mating, each female lays 1-5 eggs at a time before again feeding and mating until a total of 40-60 eggs are laid 2-4” under the soil surface in grassy areas. Most are laid by mid-August. The eggs hatch after 8-14 days, and the young grubs feed on fine grass roots until cold weather drives them below the frost line. Most are in the 3rd instar by this time. In spring the grubs come to the surface, continuing their development and forming a pupa in an earthen cell 1-3" deep in the soil.

Japanese beetle adults are active for about 6 weeks in the summer. Since adult beetle damage is primarily aesthetic, control is not essential for survival of ornamental plants. Because they are strong fliers and frequently move about, by season’s end, adults are capable of having traveled many miles from where they lived as grubs. As a result, local beetle control does not ensure less grub damage to lawns. Likewise, local grub control will not necessarily prevent adult damage to garden plants.

Integrated Pest Management Strategies:

1. Hand-pick adult beetles. If only a few adults are present, shake plants early in the morning (~ 7 a.m.) when they are sluggish. They should be collected and dropped into a container of soapy water. Any beetles or damaged leaves will attract more insects and should be removed.

2. Biological controls. The hard body of the beetle makes them unpalatable to many predators, such as birds. However, toads, moles, shrews, and skunks are known to feed on the beetles.

To control grubs, bacterial milky spore disease (Paenibacillus popillae, formerly known as Bacillus popillae) can be applied as a dust for Japanese beetle grub control and must be ingested by the grub to be effective. Once inside the digestive tract; spores reproduce within the grub, eventually turning it an opaque milky white before killing it. Spores then disperse into the surrounding soil and can persist for many years, but do not spread until live hosts are present. The more grubs, the faster it spreads. Milky spore infects only Japanese beetle grubs and has no effect on beneficial organisms. Its efficacy is questionable and the spore count has to build up for 2-3 years, during which time no insecticides may be used. More research needs to be done.

Other biological controls are parasitic nematodes that need to be applied when the grubs are small, with irrigation before and after application. Species of Heterorhabditis are said to be more effective than strains of Steinernema carpocapsae.

3. Use trap crops. When practical, crops which are highly favored by Japanese beetles can be used to draw the adults away from other crops. Trap crops include borage, white geraniums, grape vines, evening primroses, and zinnias. The beetles can be collected or killed on the trap crops. This method provides early detection and can limit the amount of spray needed to control the insects.

4. Select resistant plants. Among trees and shrubs are: ash, dogwood, American elder, shagbark hickory, holly, red and silver maple, oaks, pear, white poplar, sweet gum, and tulip tree; euonymus and common lilac.

5. Avoid planting the most susceptible plants: roses, grapes, and rose of Sharon; apple and crabapple, mountain ash, grey birch, American and horse chestnut, elm, linden, Japanese and Norway maple, London plane tree, Lombardy poplar, Prunus (cherry, etc.), sassafras, and black walnut.

6. Pheromone traps. Research has found that pheromone traps attract many more beetles than they catch and probably do more harm than good to plants in the beetles’ flight path and near the traps. Their use is not recommended.

7. Use insecticides if necessary.  To control adults, safer option insecticides like pyrethrum or Neem can be applied in two applications, 3 to 4 days apart, to control the problem. If something stronger is needed, acephate or malathion can be used. Other registered insecticides include permethrin, bifenthrin, and cyfluthrin. With any insecticide, efficacy will be decreased if there is heavy rainfall shortly after application of the chemical. 

To control grubs present in damaging numbers, imidacloprid applied in June and July (20 days before anticipated Japanese beetle adult egg-laying activity) has enough residual to kill new grubs coming to the soil surface to feed in late July through August. It is important to apply evenly over all the ground and to water it in well. These chemicals will also be effective on other grub species when applied at the appropriate times. 

Organic Strategies

Strategies 1, 2, 4, 5, and 6 are strictly organic approaches. For an organic approach to Strategy 3 consult the Organic Materials Review Institute (OMRI™) for appropriate Neem products.

Pesticide Disclaimer: 

Always follow the product's label and ensure the product is effective against Japanese beetles. Not following the pesticide label before usage is a violation of federal law.