Japanese Beetles Observed Near the "East Coast" of Illinois

June 25, 1999

· Japanese beetles have been observed in corn in Lawrence County.

· Watch for Japanese beetles in both corn (especially during pollination) and soybeans.

We've discussed Japanese beetle grubs in previous issues of this Bulletin, but now it's time to talk about the showier and usually more noticeable adult, the insect that seems to scare some folks. Japanese beetles begin to emerge at about the same time as corn rootworm adults emerge, so it is not surprising that someone has observed them already. Rick Scranton with Monsanto Company found about three to four Japanese beetles per plant in a cornfield in Lawrence County on June 21. The corn was at growth stage V-10 through V-12. Although Rick did not see any apparent feeding injury to the corn leaves, the presence of so many beetles bears watching as the corn gets close to tasseling and silking.

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 seldom is economically important. More importantly, Japanese beetle adults may clip a sufficient number of silks to prevent proper pollination.

If Japanese beetles are found in a cornfield, five randomly selected plants in each of five areas of the field should be examined and the number of beetles found on each plant should be recorded. The length of silks remaining on each plant and the maturity of the silks (i.e., no silks, green silks, some brown silks, or all brown silks) should be estimated. Control with an insecticide may be warranted if silks are clipped to less than 1/2 inch, fewer than 50 percent 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 are Penncap-M* at 2 to 4 pt per acre, Sevin XLR Plus at 2 to 4 pt per acre, and Warrior T* or 1E* at 2.56 to 3.84 oz per acre. (The use of products followed by an asterisk is restricted to certified applicators.)

Japanese beetles have a wide host range and feed on many other plants, including soybeans, fruits, and flowers. As you begin to increase the frequency of scouting in soybeans, keep your eyes peeled for Japanese beetles. Don't overreact when you find them, but don't forget about them, either.--Kevin Steffey

Author: Kevin Steffey