Japanese beetle traps are sold commercially and marketed to gardeners with roses and other susceptible plants. I cringe every time I see a Japanese beetle trap hanging in someone's garden.
Most Japanese beetle traps contain two chemical attractants: a sex pheromone, and a floral lure. Japanese beetles spend their days feeding in groups and mating, and the two attractants in combination do an effective job of luring beetles in large numbers. So why is this bad?
Studies have shown that these pheromone lures attract far more beetles than they actually trap. In other words, when you hang a Japanese beetle trap in your yard, you're inviting every Japanese beetle in the neighborhood to your yard, but only a percentage of these beetles will end up in the trap itself. The beetles that avoid the trap will now treat your landscape as a buffet.
You want the beetles to leave your yard, not come flying in by the thousands. You are better served by a Japanese beetle trap if you present it to your neighbors as a gift. Let them hang it in their backyard, so your beetles move to their garden. Or, you've got to hold a neighborhood meeting and convince everyone to hang beetle traps, to stop the migration from yard to yard.
Still, Japanese beetle traps aren't entirely without merit. They can be used effectively as a survey tool, to determine whether the numbers of Japanese beetles in an area warrant some kind of control. They also work well for managing isolated populations of Japanese beetles.
In most home garden scenarios, Japanese beetle traps are not effective for controlling these pests. For information on what does work, read How to Control Japanese Beetles.