Japanese Beetle Densities Remain Impressive: Don't Forget to Monitor Fields for Silking Clipping

July 7, 2000
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. Like many of the other grub species that have plagued Illinois producers this spring, this pest promises to be no exception. 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 August, producers will increasingly turn their attention to soybean fields and defoliation caused by this insect pest.

As Kevin Steffey indicated in last week's Bulletin (issue no. 14), Japanese beetle adults were first observed this season in Madison County on June 22, very similar to last year's first sightings of this insect pest. In general, only producers in eastern Illinois have had severe problems in cornfields and soybean fields with Japanese beetles. However, with each successive year, we seem to have increasing numbers of reports from producers in southwestern counties who are now battling this insect. As Kevin suggested, Japanese beetle adults seem to be colonizing field crops that are adjacent to virtually any major metropolitan area of the state. Dale Burmester, with Gateway FS in Randolph County, can attest to the invasion of these insects into field crops of southwestern Illinois. On July 5, Dale reported very significant infestations of Japanese beetle adults in some cornfields with densities as great as 8 to 12 adults per plant with silk clipping progressing in earnest. If silks are clipped to less than 1/2 inch in length, fewer than 50% of the plants have been pollinated, and three or more adults per ear are found, a rescue treatment should be considered. Dale Burmester's observations would suggest that scouting efforts should be well under way for Japanese beetle adults during this silking and pollination period of corn development. After anthesis, corn is less susceptible to damage; however, soybean fields will remain at risk throughout August. Insecticides labeled and economic thresholds for both crops were presented in last week's Bulletin. Let us know what local densities of Japanese beetles are like in your respective areas of the state.--Mike Gray

Author: Mike Gray

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

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