Japanese beetles are the insect bullies of the corn and soybean field. The oafish beetles make a big show by mostly munching around field edges, but when push comes to shove, they don’t always inflict enough damage to be economic.
Still, their taunting can make it tempting to do some revenge spraying, says Nick Seiter, University of Illinois entomologist. However, he says there are times to simply turn your back on the beetle activity.
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An insecticide may be warranted if scouting finds 20% or greater leaf defoliation for postbloom soybeans or 30% or more for prebloom soybeans. “Soybeans are just really good at overcoming this kind of damage,” Seiter notes.
Silk clipping is the concern in corn. It’s time to spray if you find three or more beetles per ear, and silks clipped to half-inch or less in a field that is still pollinating. “A lot of times, we find them feeding when silks are already brown and pollination complete,” he says.
Scout before using treatments, Seiter recommends. “It’s an insect that is easy to control, but doing so doesn’t prevent reentry of the insect. We want to make those treatments only when they are needed and economic.”
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