Here are a few tips to successfully controlling this pesky insect. Carbaryl (Sevin), malathion, imidacloprid (Merit) are good choices. Many of the newer lawn and garden multi-insect products are also effective. Pyrethroid containing chemicals are slightly more persistant. Spinosad and Neem based products give a little protection. Roses or shrubs may also be protected by covering with light netting. Handpicking adults from plants is an almost hourly battle. Homemade concoctions and blended beetle cocktail repellants are mildly effective, and may need reapplication every one or two days.
Japanese beetle traps may catch up to 75% of the beetles that approach them. Traps may lower beetle populations from 30% (1 trap per acre) up to 39% (10 traps per acre) if placed throughout a neighborhood. The trapped beetles must be emptied from the traps every one to two days to prevent them from rotting and releasing ammonia which is repellant to other Japanese beetles.
Traps are commercially available. Homemade traps are also effective in catching beetles if baited with phenylethyl proportionate plus eugenol lure available at garden centers and hardware stores. The traps are much more effective in attracting Japanese beetles than in trapping them. Consequently, traps should be placed as far away from the plants to be protected as possible. If traps are used, place far away from susceptible plants. Traps, alone, are not likely to give satisfactory protection to plants being eaten by adult Japanese beetles and pesticides may be required, anyway.