Japanese beetle problems started much later than last year, although their emergence this year is probably close to the normal time.
They are easy to find in many soybean fields, and are very numerous in spots. One thing to keep in mind is that the damage in soybeans usually looks worse than it actually is.
The threshold for spraying Japanese beetles and other defoliating insects is 20% defoliation during flowering and 30% defoliation prior to flowering.
Below is a picture showing about 20-25% defoliation that Marlin Rice took a few years ago:
Most people have a hard time waiting until they see this much damage before pulling the trigger, and many would likely call this 50%, rather than 20-25%. The longer you can hold off spraying, the more likely you can get by with one application and perhaps also control other insects later in the season. Most insecticides have good knockdown of the beetles but poor residual, so populations re-bound a week or 10 days after the insecticide is sprayed. Spraying when it is not needed can actually cause outbreaks of soybean aphids and spider mites by killing off the beneficial insects that keep other pest populations in check. It’s hard to believe we could have a spider mite problem with the way this season started, but if the dry weather of the past couple weeks continues, we could be talking about spider mites in another two weeks. So far soybean aphid numbers have been low, but now is the time to be scouting for them as the numbers usually increase in late July and early August.
Corn fields should be scouted for the Japanese beetles when they start to silk. The beetles are not very good flyers, so the damage in corn is often the greatest in the outside dozen rows or so. They aren’t going to fly to the middle of the corn field if they don’t have to. If silks are being clipped to within half an inch of the ear before pollination is complete, it may pay to spray the beetles in the corn, at least along the field margins.