by: Dean Gunderson
To trap or not to trap, that is the ever present question when it comes to Japanese beetles. The shiny green Japanese beetle was introduced to the northeastern United States from East Asia in the early 1900’s and has made its slow march south and west ever since. It has been in the St. Louis region for quite some time now and most gardeners have had at least one run in with this pernicious pest. Although it doesn’t feed on many vegetable crops it likes quite a few fruit trees and shrubs. These voracious eaters can defoliate whole plants if allowed. The way they eat leaves is pretty characteristic as when they feed on leaves they will eat everything except the veins of the leaves leaving a skeletonized leaf behind.
The hardest thing about Japanese beetles isn’t identifying them or figuring out what damage they do, it’s how to get rid of them! Since they are an invasive species they have very few predators so their populations can get quite large and controlling them is left to the gardener or orchardist.
But what is the best way to control the population? You could, of course, use synthetic insecticides or organic broad spectrum insecticides such as spinosad or pyrethum. However, using these as the main control would require multiple applications over several months and this option will have a negative impact on insects other than Japanese beetles including beneficial insects and pollinators.
The main tactic recommended by those who want to avoid using insecticide sprays is to knock the Japanese beetles off the leaves of the plant into a container of soapy water below where they will drown. Although this tactic works on small garden plants and for a small infestation, it’s not a feasible option if they are eating your apple or cherry tree since you can’t reach the top, where they usually start feeding on a tree. It’s also not very feasible if you have a major infestation. I can personally attest to a time when I had it on some hazelnut bushes and after knocking off the beetles into a bucket of soapy water 5 times in 1 day there were just as many at the end of the day on the bush as there were at the beginning.
There are other options for control that are less work and non toxic to beneficial insects like milky spore, a natural soil dwelling bacteria that can kill a significant amount of the Japanese beetle grubs in the soil before they emerge to eat your plants. This bacteria can be very effective at controlling Japanese beetles long term but can also have some problems. First it’s a long term tactic. If you have Japanese beetles feeding on your plants right now milky spore isn’t going to do anything to stop that. Also, it doesn’t control those flying in from neighboring properties as Japanese beetles can fly several hundred yards (1). This means for those in urban and suburban areas milky spore will only have limited success unless you get neighbors to join in. Those with large properties would typically have better success with milky spore but then there is the issue of cost. Although milky spore can remain effective for 10-20 years once applied, it can cost as much as $35 for enough to treat 2,500 square feet. So for a large property it can get costly.
The ultimate control is if there could be an insect that naturally feeds on the Japanese beetle. In fact, two predatory insects that feed on Japanese beetles have been found and introduced to the United States for just such a use; Tiphia vernalis and Istocheta aldrichi. Tiphia vernalis has been found to parasitize up to 58% of Japanese beetle grubs killing them before they emerge. Istocheta aldrichi has been found to parasitize up to 20% of adult Japanese beetles killing them before they could lay eggs. So together they could theoretically reduce the Japanese beetle population by almost 70% (2,3). Alas, although these have been introduced and established in the United States neither have made it to the St. Louis region yet and you can not purchase them yourself. So for now we will just have to hope they make it here one day.
So then again the question is what option is there if you don’t want to use insecticides, have a large infestation, and milky spore isn’t feasible for you? The answer to that question is the much maligned Japanese beetle trap.
No, not those little flimsy ones with the small bag that they sell at most hardware stores in the summer. I’m talking about a big mass trap. Many say never use the traps because they attract more to your yard then they catch. This is true if the trap is too small (as most of those bag traps are) or are not used correctly, but if used properly studies have found they can reduce the numbers to the point that they are not an issue. One study even found a 97% reduction in Japanese beetle feeding on the plants they were trying to protect (4,5).
The disconnect between these studies that find them to be effective and many people’s personal experience saying they are not effective seems to be trap size and placement. The problem is that many put the traps by the plant they are trying to protect. Then because they only have one or two traps and the trap has a small bag, it will fill up in a day or sometimes even a couple hours. What this means is that you are putting something that attracts Japanese beetles right next to the thing they are already attracted to and like to eat. So if they are flying toward the trap and run into the plant first they are going to eat that plant. Many will still be caught in the trap when you place it by the plant you are trying to protect. But, as soon as the trap is full they will fly on over and eat your plant you so kindly lured them to. So unless these traps are being emptied often so that they are never full, than they are certainly bringing more to the area than they capture.
The University of Missouri and Lincoln University have found two things that are key to making the traps effective. What they say is that you must make sure you have a large container to trap the beetles in. The second important step is to not put it by the plants but instead, around the perimeter of the area you are trying to protect. You also want to make sure that the traps are spaced no further than 200 feet apart so that you have a continuous perimeter of the lure scent to attract the beetles. By placing them in a perimeter around your property it means that Japanese beetles emerging from the soil are attracted to the edge of your property instead of towards the plants you are trying to protect. Also, those flying in from neighboring properties are intercepted and trapped before they get to your plants. If you have a relatively small property, such as a regular city lot, with the 200 foot spacing you may only need one or two traps. You would place the traps on either end of your property and or the place farthest from the things you are trying to protect. Having a large container to hold the beetles that fall in the trap helps deal with the issue of them filling up too fast and makes it more feasible to empty the containers as needed. Or if you use a really big container you might only need to empty it at the end of the Japanese beetle season.
When they need to be emptied there are a few tips to keep in mind. When you empty the container it is best to dump it into a container of soapy water to drown any that are still alive. It is also best to empty the traps before 9:00 am or after 6:00 pm because they fly during the day. Before or after those times they rarely fly, which makes it much easier to empty the traps. Once the Japanese beetles are all dead you can then decide what to do with all that organic matter. They can be put in your compost bin or just pour them on the ground at the base of your fruit trees. If you then put wood chips or other organic matter over them they will compost nicely and the bugs that were eating your fruit trees are now fertilizing your fruit trees. It is important to know though, that because they are very high in nitrogen that they smell pretty strongly once they are dead. So if you find the smell to be a problem, covering them with wood chips or throwing them away might be your best option.
Lincoln University made instructions on how to make a mass trap like the trap described here. You can find the instructions to make your own here. These instructions show attaching the trap to a large outdoor trash can but you can use any lidded container. We have had success with lidded 3 and 5 gallon buckets. These smaller containers are much easier to empty, move around, and store when not in use but need to be emptied more frequently. Our adapted instructions using a smaller container can be found here. If you would like to make your own, you can buy the tops to the traps at our store online for pickup. We also sell the dual lures which are the most effective lures for trapping Japanese beetles and are rarely sold in stores. These are great if you want to make your own traps and in future years you will be able to reuse the trap you made and just have to buy the lures.
It is also important to note that for best results the traps should be put out as soon as you first notice the first Japanese beetle. Although the traps are really good at intercepting Japanese beetles it’s harder to entice them off of something they are already feeding on. So if you have plants that are heavily infested with Japanese beetles the best option is to place your traps and then either remove the Japanese beetles by hand or by knocking them into soapy water or by using an organic insecticide like spinosad. If you have to use the insecticide, as long as the traps are already placed you should only have to use it once.