Japanese Beetles on the Move

Beware of Japanese Beetles

Reports of large captures of Japanese beetles in University of Missouri traps in NW Missouri are being reported, so I encourage farmers in Fremont, Page, Taylor, Ringgold, Decatur, Mills, Montgomery, Adams, Union, and Clarke counties to begin scouting. Keep in mind that adult Japanese beetles have brightly colored, metallic green bodies and copper colored wing covers. They also have white tufts of hair on the sides of their body. Japanese beetles only have ONE generation per year; however, they appear over a 4 to 6-week period. They need approximately 1030 GDD for emergence, which we have reached. They will appear until 2,150 GDD. Since April 1, 2026, southwest Iowa has had around 1500 to 1600 GDD accumulation. 

An interesting fact about Japanese beetles, is that females usually appear first and generally are larger than males. During their adult life, the female will alternate between feeding, mating, and oviposition of eggs in the soil. She may do this over a dozen times, laying approximately 20 eggs each time she returns to the soil. She will typically lay eggs next to host plants like soybeans and corn. The females typically prefer soybeans over corn, and this may explain why we see them in corn fields first in rotated fields. 

Crop damage is leaf defoliation to field crops, as well as horticultural crops and landscaping. Corn in drier conditions has more potential for yield loss due to drought stress and silk clipping during R1. Soybeans are more tolerant of leaf defoliation; however, later season defoliation in the later reproductive phases is more detrimental to yield. Studies suggest that 28% defoliation in the vegetative stage had little to no yield loss however, when this same level of defoliation occurs in August can reduce yield be 17%. Timely insecticide applications are critical to maximize effectiveness and minimize potential yield loss.

https://doi.org/10.1093/jipm/pmz009