There are a variety of Japanese beetle traps that can be purchased to aid in Japanese Beetle control. The basic concept linking the bag and the plastic container varieties is the use of a natural sex attractant along with a floral lure. Since Japanese Beetles feed on plant life, attractants need to be pheromone or plant related.
Since the contents of the traps are attractants, you want to be careful to not place them in a way that would attract unwanted beetles to plants you want to protect. In other words, do not use these as a preventive measure to try and stop them from ever coming.
For plants that you want to save from Japanese Beetle feeding that already have them in or on it, you typically want to place the traps about 30 feet from them downwind. This will help to draw them off and away from the plant you are focused on and intercept their flight path.
The other part of the equation is to try and determine where the beetles are coming from. If they are coming from an area off of your property that you can identify, place the traps between that source and the plants you are aiming to protect. It may take a bit of experimentation, but ideally the closer to the edge of the property as long as it isn’t obstructed by a fence or other object, the better.
The effectiveness of these products depends on how accurately instructions are followed, environmental factors such as size of population in and around the yard, attention to detail, and ability to adjust to changing circumstances. Some properties find large amounts of success in these devices and others tend to see no improvement to worsening of the situation. Keep in mind that results may vary one way or the other from one year to the next.
As with most things, the effectiveness typically increases as it is used as part of an overall strategy rather than the sole method of control.