Traps are ineffective in controlling Japanese beetles

Gardening in Gwinnett

In June, the Gwinnett County extension office receives numerous calls from homeowners concerning Japanese beetles eating their ornamental plants.

The adult beetles feed heavily on the fruit and foliage of more than 275 kinds of trees, shrubs, flowers and other plants. Their larvae (grubs) feed on the roots of turfgrasses in the fall. They are related to

June beetles, and they're brilliant, metallic-green insects with coppery brown forewings.

The Japanese beetle (Popillia japonica) was introduced into the United States in about 1916. According to the University of Georgia, they have since spread to infest much of the East Coast from Maine to northern Georgia, and inland as far as Indiana and eastern Kentucky.

In Georgia, these pests are found as far south as Macon and are abundant around the Atlanta area. Adult beetles emerge in May and June and remain active for four to six weeks. They lay eggs below the soil surface, the larvae hatch and migrate to turf root zones, where they begin to feed. They move deeper in the soil in the winter, and pupate in the early spring.

Having a drought during the egg-laying and larval hatching periods can kill many young grubs, but it's insufficient to keep population levels low enough to prevent damage to susceptible ornamental plants.

Often, people call the extension office asking about Japanese beetle traps. Unless many of the traps are concentrated in a specific area, they do not provide control. In fact, a single trap can attract large numbers of beetles, resulting in worse damage than if you didn't use traps at all. Placing a fine net over the susceptible plants can reduce the infestation.

Several chemical pesticides can be used to control the insects. Insecticides containing carbaryl, such as Sevin, and imidacloprid, such as Merit, are effective. Chemicals containing pyrethroid are slightly more persistent. Newer lawn and garden multi-insect products also are effective in controlling Japanese beetles.

Homemade remedies and blended beetle cocktail repellants give minimal control and may need reapplication every one or two days.

Japanese beetle grubs are C-shaped and white to dirty white in color, with brownish head and legs. White grubs only present a problem when they occur in high numbers. Scout for the pest by digging under a section of turf 1 square foot in size and count the grubs you find. Do this in several places and average the number of grubs you find per square foot. If the number is more than 5 to 10 in non-irrigated turf or 20 in irrigated turf, you may need to treat.

Insecticides containing Bacillus popilliae (milky spore disease) target the grubs and do not infect mammals, birds or fish. However, it is not overly effective and should be used in conjunction with other control measures. Several chemical pesticides, like those containing imidacloprid, carbaryl and trichlorfon, are labeled for use on home lawns. After applying these chemicals, make sure you water the lawn thoroughly. Follow all label directions and safety precautions when using insecticides.

Timothy Daly is an agricultural and natural resource agent with the Gwinnett County Extension Service. He can be reached at 678-377-4010 or timothy.daly@gwinnettcounty.com.