The ping, ping, ping on the windshield you may have noticed this past week is the announcement that Japanese beetle adults are back in action. Southern Illinois has seen tremendous numbers of the beetles earlier in the season. In our area, with our history of Japanese beetle, it is now necessary to keep a close eye on the damage from beetle feeding to both soybeans and corn.

Japanese beetles emerge over a long period of time. Typically adult emergence, in our area, starts around the third week of June and extends into about the last week of July. The larval (white grub) stage can and does cause root injury but lets confine this discussion to the adult beetle.

Japanese beetles feed on more than 300 host plants, including soybean and corn. Adults are attracted to flowering plants, but will feed on non-flowering plants as well. This time of the season we worry about defoliation from these beetles on soybean as it enters the critical flowering period. Although typically we are only concerned about Japanese beetle on corn interfering with pollination by clipping the silks, some attention should be paid to just defoliation when the numbers of beetles is enormous and foliar feeding is severe.

Leaf damage from the beetle looks like a large network of holes over a portion of the leaf. It is usually not hard to attribute the damage to the beetle at all because they will be present in big feeding globs near the damaged tissue. This damage is already present in some area fields. When scouting soybeans, you will want to estimate the defoliation in 5 different areas of a field. Insecticide control may be justified if you have reached 30% defoliation prior to flowering, or 20% defoliation between bloom and pod fill. There are a number of effective insecticides available for management.

For corn, if the numbers of beetles are huge prior to tasseling and silking do keep an eye on the amount of defoliation. How much? Good question. If we make the analogy of how much tissue removal can a corn plant take and not lose yield in the critical period of two weeks before tasseling to two weeks after tasseling due to a fungal leaf blight infection, then the percent is 15%. Defoliation due to insects is not exactly comparable to disease infection however; one would expect a reasonably similar reaction to tissue removal. If the corn makes it to silking without a lot of damage then scouting for silk clipping will be necessary. When scouting during silking, note the percent of silked plants, the color of the silks and the number of beetles per ear tip on average. Insecticide treatment may be justified to protect silks if there are three or more Japanese beetles per ear on average, they are clipping silks and pollination is not complete. This is not a hard and fast threshold though and you will need to make your decision based on the condition of the field as well until a more precise threshold is determined.