By Mike Gray, Entomologist
University of Illinois



In mid-June I reported that Japanese beetles were beginning to show up in many fields across Illinois. Three weeks later, infestations of this pest are common, particularly in border rows of both corn and soybeans.



Dale Baird, crop systems Extension educator, indicated that he caught more than 100 Japanese beetles in a trap located in Lee County on June 27. On July 6, the same trap caught 1,244 beetles -- an impressive increase! Similarly, Robert Bellm, crop systems extension educator, has reported heavy captures in southwestern Illinois: June 29 -- 2,027; July 1 -- 2,689; July 3 -- 1,467; July 6 -- 1,800. Chris Kallal, local field adviser for Monsanto, observed considerable feeding on corn leaves in a Mason County field near Manito on June 27.



Economic thresholds in soybeans are more clearly defined for insects, and rescue treatments should be considered when defoliation reaches 30 percent before bloom and 20 percent between bloom and pod fill.



In corn, economic thresholds for defoliation are anything but clear. In general, corn can withstand considerable foliage injury before a treatment is required. For example, the established economic threshold for fall armyworm injury is when 75 percent of plants have whorl damage and worms are present. Other factors that can help in making treatment decisions for insect defoliators in corn include hail injury estimates and predicted yield loss assessments.



A good reference on this topic is the USDA Federal Crop Insurance Corporation's Corn Loss Adjustment Standard Handbook -- 2007 and Succeeding Crop Years (FCIC-25080, 11-2006) (Adobe PDF). According to this reference (page 68), if 10 percent of the leaf area is destroyed on tassel stage corn, 3 percent of production will be lost. If 25-percent defoliation occurs, 9 percent will be lost.



I encourage readers to review the USDA handbook before making any treatment decisions regarding defoliation injury in corn. Another point to keep in mind: Japanese beetle infestations are often clumped along field edges, so don't assume that an entire field has densities equal to those in field margins. Rescue treatments may be required only in border rows.



In my estimation, the primary threat that Japanese beetles represent in corn is silk clipping during pollination. Producers should consider a rescue treatment during tasseling and silking if there are 3 or more Japanese beetles per ear and pollination is not complete. Abundant soil moisture and rapid silk growth should be taken into consideration in making treatment decisions.



SOURCE: University of Illinois.