Japanese Beetles

by Karen Weiland

Japanese beetles are one of the most aggressively damaging insect pests of landscape plants and turf grass.  Japanese beetle(Popillia japonica) larvae Japanese Beetle Damageare 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 4 to 8 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 Dead grass and grubsway 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.

It is important to know if grubs are present and at what developmental stage they are in when an insecticide is chosen and applied.    Apply an insecticide to your turfgrass only if grubs are present, otherwise you are just wasting your time and money.

The adults emerge from the soil in July and their activity is at its peak for a 6 to 8 week time period.  The timing of pesticide Japanese Beetleapplication 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.  Also, because they are more susceptible to insecticides during this time, early August is the most beneficial time to have products in place.   Always be sure to read and follow thoroughly, all directions on the product label.

There are several different kinds of applications available to either kill and/or repel Japanese beetles or grubs.  Curative applications would include using a product with the active ingredients of carbaryle  or trichlorfon.  These are designed to control existing populations of grubs/beetles regardless of how mature they are.  A preventive application would include using a product with the active ingredient of imidacloprid.  This type of application is the use of persistent chemicals that will stay active in the soil for long periods of time. Products in this category may provide very good grub control even if applied several months before egg hatch (which occurs in early August) but note that they are more effective the closer to the egg hatch date that they are applied.  The last application is termed alternative.  Milky Spore and parasitic nematodes fall into this more organic category.  Both of these products will work only on japanese beetle grubs, not the beetle itself.   These are just a few of the choices available.  The Purdue Extension publication E-75-W includes a more extensive list of products to combat japanese beetles and the grubs.  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=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!

More information about gardening and related subjects is available online at www.hort.purdue.edu/ext/garden_pubs The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange Co., 636-2111 in Noble Co., 925-2562 in DeKalb Co and 668-1000 in Steuben Co.