During the month of June, Japanese beetles feed on 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, colored a brilliant metallic green with coppery brown forewings. They feed on the upper surface of leaves and consume the tissue between the veins, causing a skeletonized appearance to the damaged leaves.
Adult beetles emerge in May and June and remain active for four to six weeks. The insects live from 30 to 45 days. The females lay 40 to 60 eggs below the soil surface. The grubs hatch and begin to consume plant roots and organic matter. They move deeper in the soil in the winter, and pupate in the early spring.
People often 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 Japanese beetles, resulting in worse damage than if the traps were not used. 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, malathion and imidacloprid are effective. Chemicals containing pyrethroid are slightly more persistent. Newer lawn and garden multi-insect products also are effective. Multiple applications may be necessary when adults are actively feeding.
The beetles usually consume plant material in masses and prefer plants in direct sun. They emit pheromones that attract other beetles to feed and mate, increasing the number coming to feed on the plants. If they are not many in number, you can remove them by simply picking or shaking them off and dropping them in a bucket of soapy water.
Japanese beetle grubs are
C-shaped, 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 pests by digging under a section of turf one 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 five to 10 in non-irrigated turf, or 20 in irrigated turf, you may need to treat. Insecticides containing milky spore disease target the grubs and do not infect mammals, birds or fish. However, it is not overly effective and should be used with other control measures. Several chemical pesticides, such as those containing imidacloprid, carbaryl, and trichlorfon are labeled for use on home lawns. After applying these chemicals, make sure you water the lawn thoroughly.