The Management of Japanese Beetles Begins Soon. - U of I Extension

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The Management of Japanese Beetles Begins Soon.

This article was originally published on March 25, 2010 and expired on May 25, 2010. It is provided here for archival purposes and may contain dated information.

Here are some tips from Dr. Phil Nixon, Extension Entomologist, University of Illinois, for starting early to help keep Japanese beetle damage as low as possible this summer.

Application of imidacloprid (Merit, Bayer Advanced tree and shrub insect control, and other brand names), and other systemic insecticides are effective in controlling Japanese beetle adults. Although the imidacloprid occasionally doesn't work in a tree to control this pest, it does over 80 percent of the time if applied correctly. However, a soil application of imidacloprid takes up to 2 months to move completely up to the leaves of large trees. With Japanese beetle emergence typically ranging from the third week of June in southern Illinois, to the fourth week of June in central Illinois, and the end of June or first week in July in northern Illinois, now is the time to think about a systemic treatment to achieve control in that manner.

Imidacloprid can be soil-applied either as a drench or by injection. Because imidacloprid is easily tied up on organic matter, mulch and other dead organic matter must be removed from around the base of the tree before a drench application is made. Removal of turf around the tree would also be required for a drench. Soil injections should be made deep enough to get below mulch, turf thatch, and other organic matter, but not deeper than 3 to 4 inches. Apply to the soil within 1 to 2 feet of the trunk, where the greatest concentration of fine feeder roots is located.

Northern Illinois had very high numbers of Japanese beetle adults last year, whereas many areas of central Illinois had low numbers of the beetles for no obvious reason. Southern Illinois has rather typical numbers of the beetles. Last fall and winter was close to ideal for the survival of Japanese beetle grubs in the soil, so high numbers of beetles are expected this summer.

Less than 11 inches of rainfall from July through fall reduces the survival of Japanese beetle larvae, and most areas of Illinois received at least this if not much more rain. Although the winter seemed to be severe to us, and there were extended periods of subzero temperatures, insect numbers were probably not reduced to a great extent. Increased snowfall insulated Japanese beetle grubs from experiencing any long-term, deep-frozen soils, which can reduce their numbers. In addition, there were no winter thaws or early spring warm weather to possibly fool the grubs into burrowing close to the soil surface too early. The winter and spring conditions were almost ideal for most other overwintering insects in Illinois this year.

Now is not the best time to control Japanese beetle grubs however. The grubs will feed for a short time this spring and then pupate. Grub control products will not be as effective when the grubs are in the pupae life stage. These products will do a good job if your turf is being severely damaged this spring and you actually find the grubs present. This will not usually be the case, but you can check your lawn if you suspect grubs. If you can pull large sections of your turf up like a carpet and see at least 12 grubs per square foot, it would be the time to treat your lawn. This threshold is the same for later in the summer when grubs will do more damage if they are present. As always, please read and follow label directions.

One last point is that just because you control grubs in your lawn does not mean you will reduce the population of the adult beetles in your yard this summer. The beetles can fly for several miles, so there will be just as many on your plants whether you treat for grubs or not.

For more information about Japanese beetle or other pests, contact your local U of I Extension office. In Ogle County call (815) 732-2191 or visit or e-mail Bill Lindenmier at

Source: Philip Nixon, Extension Specialist, Entomology,

Pull date: May 25, 2010