The beetles will drop into the container and be killed. Prized roses and ripening fruit can be protected by covering with floating row covers. Japanese beetle traps are not recommended. University of Kentucky studies have shown that if only one or two traps are used in a garden as few as 54 percent of the beetles are captured and there is a net increase in the beetles in the area around the traps. This is true even though in the study a single trap often captured as many as 20,000 adult beetles in a single day.
Traps can actually increase damage.
Traps have been shown to be only effective in community-wide efforts. The bacterial control, milky spore, sold as Doom or Grub Attack is commonly recommended to control Japanese beetle grubs. In our area, milky spore is generally not recommended since it only controls Japanese beetle grubs and not our predominate lawn grub, annual white grub. Also Japanese beetle grubs must already be infesting the turf for milky spore to work effectively. Pesticides commonly used for lawn grub controls such as diazinon will also control Japanese beetle grubs.