About four years ago I described our initial battles with Japanese beetles, and showed a picture of the Spectracide bag system that we employed. After being emptied several times, and being subjected to the winds common in our yard, that bag ripped beyond repair and forced me into a different alternative. I was pleased with the success of the lure and trap idea, but modified the concept as shown below.
The bucket lid has a 3.5" diameter hole cut in the top and the funnel is simply caulked in place. The funnel end was trimmed back so the smallest portion of the funnel is about 1" in diameter to allow the beetles to more easily enter the bucket. After four years of use I replaced the lid this year as the original Menards lid became brittle and unusable after many hours of exposure to the sun. The bucket itself also cracked at the mounting hole of the handle, but I was able to relocate the handle to a different location so I hope to get more seasons of use from this bucket.
The big question is: does this type of trap attract more beetles than it kills? A quick search around the internet (and reading the article from the Kentucky entomologists quoted above) will show that there are plenty of folks that think these traps are a waste of money and worse for your plants than if you didn't have a trap at all. Spectracide recommends placing your trap 30 feet downwind from the plant of interest, but because our wind comes from lots of different directions, and we have four different plants that attract the beetles, a perfect location on our property would be hard to identify. The image above shows the trap location that has served us well for the last four years. As you can see, none of our plants is less than 30 feet from the lure, and most don't even have a line of sight to the lure. Whether that makes the trap more or less effective, I can't say.
What I can say: the damage due to Japanese beetles on our property is severely reduced by the presence of our trap. Does the lure attract beetles from neighboring properties? Maybe, but if our plants have less damage, I don't really care whose beetles I'm killing. Are there more beetles attacking our plants because I have a lure in my yard? Judging by the damage to the plants, I would conclude that fewer beetles attack our plants since we have less damage when the trap is active.
Tips for best success with a beetle trap
1. Set the trap out early in the season. By the middle of June here in Central Illinois the beetles' damage can be visible from a distance. In my experience, luring the beetles before they can cause extensive damage will reduce the total amount of damage done.
2. Pick beetles off of plants by hand. If you find beetles on your plants, you can easily remove them by grabbing them and tossing them into a bucket of soapy water. I carry my lure bucket to the raspberry plants and hops and have it close by when I physically remove as many bugs as I can. This is easiest to do in the morning or evening as the beetle are much more lethargic and typically don't try to fly away when you grab them. If you find grabbing them unpleasant, you can position your soapy water beneath the branch and shake them into the container. With taller plants like our cherry tree, I simply try to shake the branches to cause the beetles to fly away or drop to the ground. Either way, they're likely to smell the lure and be more attracted to my trap than the tree. The maple tree is too tall, so I make no attempt to remove the beetles from that tree.
3. Remove as many beetles as possible. Several sources indicate these beetles can smell a plant that is being eaten by another beetle, so it's common to see beetles swarm a plant and ignore its neighbor. I have found that if I can remove all the beetles from a plant the beetles are much less likely to come back to that plant. Repeating the process over several days can leave a plant beetle-free for the rest of the summer even though it had been a favorite days before.
I don't have scientific test results that prove our trap is beneficial, rather only my observations. I've seen the damage that beetles can do when left unchecked, and I remember wondering if our cherry tree was going to survive their attack. Since employing our lure bucket, the only time we've had noticeable damage was when I was late setting the trap out and the beetles got a head start on me. I don't have experience dealing with the beetles in the larva stage (other than killing the ones I find when digging in the yard), but I do know the adult beetles can be controlled significantly with a lure trap like mine.