When I first started collecting hardy hibiscus japanese beetles were not in IL yet. The first year I had japanese beetles they only attacked my pussy willow and my rugosa roses.
Well, fast forward to today. The Japanese beetles are at the peak of their mating/eating season (middle of July through August) same time the hardy hibiscus bloom. And you guessed it they are all over them flowers! What a way to ruin them gigantic flowers.
To save them amazing flowers I have stumbled upon a secret. Cut them back! Cutting the hibiscus down to 6-8″ around July 4th will produce later flowering, shorter, fuller, better branched plants with more flowers and no beetles! The hibiscus at first will look like small sticks, don’t be afraid as after a week you will see tons of new growth.
The Japanese beetles will be about done when the hibiscus is ready to bloom, thus saving the flowers!