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Knowledge to Grow by Karen Weiland - Japanese Beetles

Japanese beetles are one of the most aggressively damaging insect pests of landscape plants and turf grass. Japanese beetle (Popillia japonica) larvae are a type of white grub that feeds on the roots of grass and the adult Japanese beetles eat the leaves and flowers on more than 300 plant species. The adults are about 3/8” in length and have metallic green heads and metallic copper- tan backs. The larvae are a “c” shaped white grub.

The Japanese beetle has a one-year life cycle. After emerging as adults, they basically feed, mate and lay eggs. In late afternoon, the mated females will seek suitably moist turf grass soil in which to lay her cluster of eggs among the plant roots. A female can lay 40-60 eggs during her four- to eight-week life span.  After the larvae have hatched, they start feeding on turf grass roots. In the fall after the soil temperature drops to about 60 degrees F the larvae move deeper into the soil where they remain throughout the winter. As the soil warms in the spring the larvae become active again to form an earthen cell and pupate.  A few weeks later they emerge as adults.   

Just because you have a large amount of adults feeding on your plants does not mean you have a grub infestation in your grass.  Adults will fly a long way to find food. But if you do find patches of dead grass in your yard that can be rolled back like a carpet, chances are you have a grub issue. Grubs will chew off the roots of the grass, therefore causing the grass to be unable to take up water during the hot, dry weather of summer. Starlings, crows, moles, shrews and skunks damaging the lawn may be another indication you have a grub issue.

The adults emerge from the soil in July and their activity is at its peak for a six- to eight-week time period. The timing of pesticide application is important in the control of Japanese beetle grubs. Because of their egg-laying, the best time to apply grub control insecticide is mid-July through early September using a granular insecticide applied with a spreader. Always be sure to read and follow thoroughly, all directions on the product label.

There are numerous insecticides available to either kill and/or repel Japanese beetles. Pyrethroid insecticides offer two to three weeks of protection after a single application. Carbaryle (Sevin) is another good choice but its effectiveness will not last as long as the pyrethroid. These are just a few of the choices available. When making your choice, read all label warnings and cautions.  Some products are highly toxic to bees and other pollinating insects and plants grown for food. Keep in mind, no pollinators equals no food!

Another effective but time consuming control method is to simply pick the Japanese beetle from the plant in the early morning and drop it into a container of soapy water. This would not exactly be my idea of how I would like to spend my morning, but it does work. Japanese beetle traps are not recommended as they tend to increase the damage done by the beetles by drawing them into an area in larger numbers than can be trapped.  

Good luck and as always, Happy Gardening!

The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange County.