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Japanese Beetle Reports Common in Soybeans and Cornfields

June 28, 2002
During the past week, we've received several reports of Japanese beetle infestations in corn, soybeans, and even rose gardens. On June 25, Kevin Black, with Growmark, observed an infestation of Japanese beetles clipping silks within a cornfield located near Waterloo, Monroe County. On June 18, Omar Koester, crop systems Extension educator, Randolph County, noted that Japanese beetles were common in many local soybean fields and nearby suburban residents also were finding these attractive insects "munching" on their roses. Shawn Jones, field sales agronomist with Pioneer/DuPont, reported on June 25 that "moderate" numbers of Japanese beetles were showing up in cornfields located near Macon, Maroa, and Mt. Zion (all three communities located in Macon County). In addition, Shawn indicated that Japanese beetles could be found on some trees within residential areas of Decatur. Although many insect species are more selective when it comes to their diet, Japanese beetles are not "picky eaters."

The Japanese beetle was first reported in the United States (New Jersey) in 1916. This insect pest has now spread into all states east of the Mississippi River, with the exception of Mississippi and Florida. Isolated pockets of Japanese beetles also have been reported in Missouri, Iowa, and Nebraska, as well as California, where it was apparently eradicated. Although most of our readers are concerned with the injury inflicted on cornfields and soybean fields, Japanese beetles also are pests of many ornamental plants and fruit trees. In the coming weeks of July, corn growers are urged to monitor their pollinating fields for Japanese beetle adults and their silk-clipping activities. In July and August, producers also are encouraged to monitor their soybean fields for defoliation caused by this insect pest. Let's take a look at a few commonly asked questions regarding the management of Japanese beetles in corn and soybeans.

What's the easiest way to identify Japanese beetles?

Adult Japanese beetles are "attractive" insects. They are shiny metallic green and have bronze-colored wing "covers." Along the abdomen and below each wing cover, you'll find six tufts of white bristles. The adults are robust and reach approximately 1/2 inch in length. One cautionary note about the identification of Japanese beetles: Recently, Kevin Steffey observed large numbers of false Japanese beetles near Arenzville, Cass County. These beetles are usually found in areas where soils are sandy. Although false Japanese beetles rarely, if ever, cause economic damage to crops, it is important to identify them correctly because they are often confused with Japanese beetles. False Japanese beetles are about the same size as Japanese beetles, have dull, coppery brown wing covers, and are dull green near the head end of the body. They do not have the characteristic arrangement of white tufts along the abdomen, as mentioned previously for the Japanese beetle. Control of false Japanese beetles is not recommended in agricultural crops in Illinois.

How long will Japanese beetles be around this summer?

These insects complete their development in 1 year throughout most regions of the United States. In northern New England, the entire life cycle of Japanese beetles requires 2 years. Female beetles deposit their eggs within the soil as early as mid-June and continue oviposition through August. After hatch, later in the summer, the first three larval instars feed on decaying vegetation and the roots of grass species. Overwintering is accomplished by the more mature larvae that move below the frost line. In the spring, larvae move upward in the soil profile and complete their development. Pupation most often occurs in May, and adults are often noticed for the first time in mid- to late June in Illinois.

What thresholds should be used in corn and soybean fields?

Although Japanese beetles may feed on corn leaves, the most significant economic threat they represent in corn is their potential to clip silks and interfere with the pollination process. Corn leaves that have been fed on are skeletonized or lacelike in appearance. The injury is very similar to that caused by corn rootworm adults. This leaf injury is almost never of any economic importance. Before making a treatment decision, the length and maturity of silks should be estimated within a field. If three or more Japanese beetles per ear are found, silks have been clipped to less than 1/2 inch, and less than 1/2 of plants have been pollinated, a treatment should be considered. Economic thresholds for soybeans are based on the level of defoliation. A treatment should be considered when defoliation reaches 30% before bloom and 20% between bloom and pod fill.

What products are labeled for control of Japanese beetles?

In corn, the following treatments may be used as rescue treatments: *Capture 2EC (2.1 to 6.4 ounces of product per acre), *Penncap-M (2 to 4 pints of product per acre), Sevin XLR Plus (1 to 2 quarts of product per acre), and *Warrior (2.56 to 3.84 ounces of product per acre).

In soybeans, the following treatments may be used as rescue treatments: *Ambush (6.4 to 12.8 ounces of product per acre), *Asana XL (5.8 to 9.6 ounces of product per acre), *Penn-cap-M (3 to 4 pints of product per acre), *Pounce 3.2 EC (2 to 4 ounces of product per acre), Sevin XLR Plus (1/2 to 1 quart of product per acre), and *Warrior (3.2 to 3.84 ounces of product per acre).

Products preceded by an asterisk (*) are restricted-use insecticides and may be applied only by certified applicators.--Mike Gray and Kevin Steffey

Japanese beetle on corn silks.

Japanese beetles on soybean leaf.

Soybean defoliation levels.

Author: Kevin Steffey Mike Gray

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

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