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Japanese Beetle Woes Continue

July 19, 2002
Japanese beetles continue to cause problems for corn and soybean growers in many areas of Illinois. In some counties, infestations are spotty; in other counties, most fields seem to be infested. Reports of silk clipping in cornfields and defoliation in soybean fields are common, and insecticide applications to prevent further injury are warranted in many fields. One agronomist estimated that about 10,000 acres of corn and a few thousand acres of soybeans had been sprayed in Christian, DeWitt, Macon, and Moultrie counties.

We're not entirely certain how much longer Japanese beetles will afflict us, but there's some potentially good news on the horizon. Steve Engels, an agronomist in southern Indiana, reported on July 12 that Japanese beetles in his area may have declined by about 75%. He still finds them in soybean fields, but they are scarce in cornfields. Ron Hines, senior research specialist with the University of Illinois Dixon Springs Agricultural Center, has indicated that captures of Japanese beetles in traps have plummeted from 300 to 500 per day to only 30 to 60 per day. The cause for these declines is a mystery, but it's worth noting for those people who have been battling Japanese beetles since late June.

In a previous Bulletin (issue no. 15, July 5, 2002), I discussed some of the feeding preferences of Japanese beetles. By now everyone knows that Japanese beetle adults feed on leaves, tassels, silks, and pollen in corn and on leaves and flowers in soybeans. In both crops, Japanese beetles are capable of interfering with pollination; consequently, these insects threaten yield potential. Matt Montgomery, Extension unit educator in crop systems in Springfield, took some photographs that reveal the end results of Japanese beetles affecting kernel set in corn. In a field near Illiopolis (Sangamon County), silk clipping by these beetles resulted in ears on which kernels developed on only half of the cob. In another field in which silk clipping was less severe, only the tips of the ears lacked kernels. This evidence should be an ample reminder for people to monitor for Japanese beetles in late-planted corn that has not begun to pollinate.

Japanese beetles clipping silks on a corn ear. (Photo courtesy of Matt Montgomery, Extension Unit Educator in Crop Systems, Springfield.)

Poor kernel set after significant silk clipping by Japanese beetles. (Photo courtesy of Matt Montgomery, Extension Unit Educator in Crop Systems, Springfield.)

Kernel set after slight silk clipping by Japanese beetles. (Photo courtesy of Matt Montgomery, Extension Unit Educator in Crop Systems, Springfield.)

Japanese beetle injury to either corn or soybeans will add to the other stresses (e.g., moisture stress) that crops are currently subjected to in some areas. Therefore, consider adjusting economic thresholds accordingly. In some situations, it's appropriate to lower economic thresholds for insect damage when plants are suffering from other stresses. However, you also must consider potential yield when making such a decision. The potential for lower yields usually results in an increase of economic thresholds. So, use your best judgment in each field you scout. Consider the density of Japanese beetles, amount of injury (silk clipping in corn, defoliation in soybeans), level of crop stress, yield potential, and cost of control before making a decision to treat a field with an insecticide.

Thresholds and insecticides for management of Japanese beetles in corn and soybeans were provided in issue no. 14 (June 28, 2002) of the Bulletin. Keep scouting and sending reports.--Kevin Steffey

Author: Kevin Steffey

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

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