Just a quick note that I saw mini-swarms of Japanese beetles on my lunch break today; I hadn't noticed them in Central Illinois yet. I did find lots of grubs in my yard earlier this spring, and from what I understand we're expecting to see a lot of the little buggers (ha!) this year.
Remember that Japanese beetles are attracted to each other (hence the feeding swarms we see in summer) and to damaged leaves. Early season control of adults can reduce damage over the growing season. Because the beetles are attracted to pheromones released by other beetles, we recommend removing as many live or dead beetles as possible (if you handpick, throw them into a bucket of soapy water; if you apply an insecticide, sweep or try to collect as many fallen beetles as possible).
Insecticides containing the following active ingredients are labeled for use against Japanese beetle: acephate, carbaryl, cyfluthrin, imidacloprid, permethrin, and zeta-cypermethrin. Always follow the label for any product you choose to apply, and ensure that it is labeled for use on the plant(s) you are targeting.