Hardy Hibiscus is prone to Japanese beetles. You can make a big dent in the population by picking most of them off by hand. Carry a small container of soapy water to drop the beetles into as you go. You can also try introducing Milky Spore disease purchased from a garden center. It infects the beetles and ends the problem.
There's no way to get rid of them totally in any given season but can be controlled by Applying Bayer Advanced Dual Action Rose & Flower Insect Killer (available in Ready-to-Use and Ready-to-Spray formulations) as soon as possible. This product provides up to 30 days rainproof insect protection (including Japanese Beetles) in a single application.
To protect a rose of Sharon bush against a beetle infestation, you have to maintain your garden very clean and neat. hygienic surroundings, environmental situation is vital for the well growth of flowers especially for a rose of Sharon bush. You can also fence them by the help of nets.