First Report of Japanese Beetles

June 30, 2000
Many of us will remember the very large numbers of Japanese beetles that occurred in corn and soybeans last year, especially in eastern and central Illinois. Well, we need to be ready for them again because early indications suggest they may be numerous in 2000. Robert Bellm, crop systems educator at the Edwardsville Extension center, reported the first Japanese beetles of the season in Madison County on June 22. They were feeding in corn whorls. Based on Robert's observations of numerous Japanese beetle grubs earlier this year, we might want to anticipate some fairly significant densities.

We used to think of Japanese beetles as a problem experienced only by folks in east-central Illinois. However, that has changed in recent years. Our understanding is that Japanese beetles have been reported around homes and gardens in most of the major cities in Illinois. Therefore, we should anticipate that they will show up in nearby crop fields and possibly spread from there. Let us know if and when you find these handsome, albeit irritating, insects this summer. We'll want to keep track of their distribution within the state.

We first reported Japanese beetles found in Lawrence County last year on June 21, so their emergence right now is occurring at virtually the same time. Our report in issue no. 14 of last year's Bulletin (June 25, 1999) was a herald for what turned into an onslaught in 1999; the reports of injury to crops continued well into August. So keep your eyes and ears open. A sudden and significant attack of these insects in cornfields and soybean fields always seems to catch a lot of people by surprise. And homeowners will also enter the fray when they begin finding Japanese beetles in their flower and vegetable gardens and fruit trees.

Japanese beetle on corn silks.

The adult Japanese beetle is shiny metallic green with hard, bronze-colored wing covers. Along each side of the abdomen, just below the wing covers, are six tufts of white hair. The adult is about 1/2 inch long. The beetles may feed on leaves if silks are not available; injured leaves appear skeletonized or lacy, similar to injury caused by corn rootworm beetles. However, leaf feeding is seldom economically important.

More importantly, Japanese beetle adults may clip a sufficient number of silks to prevent proper pollination. Control with an insecticide may be warranted if silks are clipped to less than 1/2 inch, fewer than 50% of the plants have been pollinated, and beetles are feeding. A rule-of-thumb economic threshold is three or more beetles per ear. Insecticides suggested for control of Japanese beetles in corn are *Capture 2EC at 2.1 to 6.4 oz per acre, *Penncap-M at 2 to 4 pt per acre, Sevin XLR Plus at 2 to 4 pt per acre, and *Warrior T at 2.56 to 3.84 oz per acre. (The use of products preceded by an asterisk is restricted to certified applicators.)

As most of you know, the greatest concern regarding Japanese beetles is their feeding, sometimes in very large numbers, on soybean foliage. Although no one has reported finding Japanese beetles in soybean fields yet, you should be aware of the threshold, just in case the beetles show up in soybeans soon. Treatment may be warranted in soybeans during vegetative growth if defoliation reaches or exceeds 40%. The threshold decreases to 15 to 20% defoliation during flowering, pod set, and pod fill. Insecticides suggested for control of Japanese beetles in soybeans are *Ambush at 6.4 to 12.8 oz per acre, *Asana XL at 5.8 to 9.6 oz per acre, *Penncap-M at 3 to 4 pt per acre, *Pounce 3.2 EC at 2 to 4 oz per acre, Sevin XLR Plus at 1 to 2 pt per acre, and *Warrior T at 3.2 to 3.84 oz per acre. (The use of products preceded by an asterisk is restricted to certified applicators.)--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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