Invasive Japanese beetle adults have begun to emerge in central Missouri. Adult beetles feed on over 300 host plant species including natural weeds, shrubs, hardwoods, and cultivated crops such as to corn, soybean, ornamentals, and fruit, often resulting in economic injury. Recent flooding throughout the state is unlikely to affect Japanese beetle populations. Beetle grubs overwinter in the upper 5-15 inches of soil and resume feeding on grass root in the spring prior to adult emergence. Although standing water on agricultural fields can reduce oxygen availability and enhance the spread of disease, it is unlikely to suppress over all beetle populations levels. During the 1940, wide scale flooding on the east coast did not affect the beetle abundance. Furthermore, adults will emerge from grasses not effected by flooding.
Adults will continue to emergence throughout early summer, with peak populations typically occurring in July. Adults can cause severe defoliation in soybean. Economic thresholds are based on defoliation. During vegetative growth, chemical applications should be considered at 30% defoliation, and 20% during reproductive stages. Japanese beetles often form aggregations in field edges, therefore border treatments may provide sufficient management, however, interior of fields should also be scouted. Late planted soybeans may reach thresholds earlier as these plants will be smaller.
Adult beetles also consume corn silks, resulting in reduced pollination and ear fill, however foliage feeding does not cause injury. Insecticide applications should be considered when silks are present, and three or more beetles are present per ear and pollination is less than 50% complete.
Researchers at the University of Missouri are currently investigating an attract-and-kill strategy to manage Japanese beetles while reducing insecticide applications. This technique uses lures containing pheromones and plant volatiles to attract adult beetles to a specific area where they can be killed.
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REVISED: February 21, 2017