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Japanese Beetles - Destructive Pest in the Garden

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By Dennis Patton, Horticulturist 

Japanese beetles have become an annual pest. These small, brightly colored beetles are ravenous feeders quickly destroying the beauty of many landscape plants. Dealing with this pest is not always easy.


How to Identify Japanese Beetles
Japanese BeetlesJapanese beetles can be troublesome on two fronts. The adults feed on a wide variety of plant materials, including rose, grape, crabapple, linden and birch. In fact, they feed on over a hundred species of plants. The grub can be a pest of the lawn, feeding on the roots.

What people detect is the adult stage, which is active for about six weeks in the summer. They can start appearing in late June and the feeding lasts into August. The adult is less than ½ inch in length, and metallic green in color with coppery wing covers. The most identifying marking is white tufts of hair that protrude from the end of the abdomen. For the average person, that means white dots on the side of the small beetle.

The larvae are a white grub that looks much like the other grubs found in our area. Identification of the Japanese beetle will require a microscope and a closeup inspection of hair patterns around its anus. I am going to assume most of you will leave anal inspection to the trained professional. I can hardly wait for your sample!

Japanese Beetle Damage
Japanese Beetles on rose buds

Damage from the adults is the defoliation of the host plant. The good news is, many well-established plants can tolerate feeding with no loss in vigor. The feeding tends to be more in mid to late summer, which means the plant has had more time to store food reserves for next year. Unfortunately, the plant loses blooms and looks ratty for a period. Light fertilization will help annuals, roses and perennials develop growth and recover.

The most severe damage is done in vegetable gardens and fruit plantings where there is a loss of production and little or no time for the plants to recover.

Controlling Japanese Beetles
Japanese Beetles in a bucket of soapy water

Options for control range from doing nothing to hand removal or chemical sprays. They often feed in clusters, so knocking the adult beetles off into a bucket of soapy water is the preferred non-chemical control. Knocking the beetles off is best done in the cool of the early morning when they are less active. They fly rapidly during the heat of the day, making control more difficult. Knocking off is a daily task to be most effective.

Chemical sprays are effective in reducing feeding.  Recommended insecticides include Cyfluthrin, Bifenthrin, and Cyhalothrin.  These products will need to be reapplied about every couple of weeks during the feeding period.  Neem Oil, which is considered organic, can be used, but it needs to be reapplied about every 3 to 4 days. The disadvantage of spraying is the removal of beneficial insects from the plant that can control other pests such as spider mites.

Japanese beetle traps are available on the market. These traps contain a lure or scent that draws the beetles to the area. If not correctly placed, they can increase the number of beetles in your yard. They are not highly recommended for a typical suburban lot. If used, set as far away as possible from the garden or plants wishing to protect.

Learn more on Japanese beetles with this resource from K-State Research and Extension.

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Have questions? The Garden Hotline is staffed by trained EMG volunteers and Extension staff who will assist you with questions.

Phone: (913) 715-7050

Email: garden.help@jocogov.org