University of Illinois

No. 17/July 17, 1997

Japanese Beetles: A Potential Threat to Corn and Soybean Fields

Robert Bellm, a crop systems educator at the Edwardsville Extension Center, indicated recently that Japanese beetles are becoming a force in many corn and soybean fields in southwestern Illinois. Japanese beetles begin to emerge in July and are evident in many fields well into August. Observations of these attractive insects are most often made on sunny days, with the beetles feeding on the sunny leaf surfaces of plants.

Most concern for this insect pest in corn centers on protecting the pollination process. A rescue treatment in field corn is normally not warranted unless three or more beetles per ear are actively clipping silks and the pollination process is not yet complete. For seed-production fields, control of Japanese beetles may be necessary if the silks on 20 percent of the plants have been clipped to a length of 3/4 inch or less, pollination is still taking place, and beetles are still present (Seed Corn Pest Management Manual for the Midwest). Insecticides labeled for use against Japanese beetles in field corn include Sevin XLF Plus (2 to 4 pints of product per acre) and Warrior 1EC (2.56 to 3.84 ounces of product per acre). The use of Warrior 1EC is restricted to certified applicators only.

Prior to the bloom and pod-fill stages of development, soybeans can generally withstand a considerable amount of defoliation (up to 30 percent) before a rescue treatment is warranted. During the reproductive period of soybean development, a 20 percent loss of foliage may warrant an insecticide treatment for this pest. Products labeled in soybeans include *Ambush 2E (6.4 to 12.8 ounces of product per acre), *Asana SL (5.8 to 9.6 ounces of product per acre), *Penncap-M (3 to 4 pints of product per acre), *Pounce 3.2EC (2 to 4 ounces of product per acre), Sevin XLR Plus (1 to 2 pints of product per acre), and *Warrior 1EC (3.2 to 3.84 ounces of product per acre). Use of an insecticide preceded by an * is restricted to certified applicators only.

Before you make any management decision, take the time to walk the entire field when you determine the average number of beetles per ear tip and when you estimate soybean defoliation. Japanese beetles commonly reach their greatest densities in the outer 12 to 15 border rows. The number of beetles found per corn ear tip or feeding on soybean leaves may decline sharply when you move farther into the field.

Mike Gray, Extension Entomology, (217)333-6652