Japanese beetles are more of a problem this year than last say University of Illinois Extension entomologists, who report 417,102 beetles caught in one trap over the course of a week. However, specialists say populations are highly variable, and are worse in some areas and less of a problem in others.
If you need to control Japanese beetles, Mike Gray and Kevin Steffey at Illinois suggest:
1) Pyrethroids kill Japanese beetles on contact, but they also are repellent to beetles.
2) High temperatures may reduce the efficiency of some pyrethroids
3) Tank-mixing different insecticides should not be necessary in most situations.
4) Assess the situation for the entire field and spray only where necessary.
5) Japanese beetles become a non-issue in cornfields after pollination is complete.