Between June and July, growers may notice small beetles with shiny copper-colored bodies and green heads popping up in their corn and soybean fields. These little nuisances are Japanese beetles — a pest that became an issue for growers last year and could potentially affect yields this year if not properly managed.
Japanese beetles emerge from hibernation as grubs in the spring as temperatures rise and then pupate into adults within four to six weeks after. Unfortunately, they do not travel alone. Where there’s one Japanese beetle, much more are likely nearby. When they feed, they emit a pheromone that attracts hordes of other beetles.
According to the Iowa State University Department of Extension and Outreach, Japanese beetles host on more than 300 plants. In soybeans, the beetles feed on and skeletonize the leaves. In corn, they can be even more damaging if they clip silks during pollination, which could affect grain fill and yield. Iowa State University recommends that treatment in soybeans should be considered if 30 percent of the leaf is skeletonized before bloom and 20 percent defoliation after bloom. For corn, foliar insecticide should be considered at tasseling and silking if three or more beetles can be found per ear, if the beetles have clipped less than a half inch of the silks and if pollination has not yet reached 50 percent complete.
I recommend to spray a Lorsban-type insecticide over the canopy if you have a high enough level of silk clipping in corn. This tactic will help you achieve good penetration and control over the beetles. I recommend the same treatment for soybeans if leaf defoliation is present before bloom and pod fill, and even after bloom. Because Japanese beetles can survive in your soil through the winter, get ahead of them this year to prevent them from coming back in the coming years.
To learn more about signs of Japanese beetles and how to protect your fields, contact your local Stine representative.