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Are Japanese Beetles Harmful?

Last Updated: 16.06.24


If you have been reading reviews of Japanese beetle traps for a while now, and are interested in how you can get rid of these insects from your garden, you may find this article to be very useful to you. To better assess which are the most effective ways of keeping Japanese beetles away from your garden, you first need to better understand how they live and what they feed on.

Japanese beetles are species of the scarab beetle. Their size is not that big, measuring about 0.6” in length and 0.4” in width. Their color is green on the head and thorax and iridescent copper on the back. Most Japanese beetle species have a total life span of about one year, while few species have a two-year life span.

Out of the total life span, Japanese beetles spend most of their lives as larvae. It takes them about two weeks to hatch from the eggs the mother beetle lays, then larvae develop over a 10 month period, and then spend their last 30 to 45 days as flying adults. Larvae beetles live in the soil and feed on the roots of plants, thus destroying the plants.


How harmful can Japanese beetles be?

Although beetles are not directly harmful to humans, they may cause significant economic damage because of their life cycle and feeding habits. A mother beetle will lay up to 60 eggs in her short lifetime. From these eggs hatch small larvae that feed on the soft roots of plants and other available organic material. Nothing wrong up to this point.

From here on, larvae will continue to develop and grow for many months to come. As they grow they start feeding on thicker and stronger roots, thus causing the plants to die. Another important aspect to be taken into consideration is the ability of larvae to hibernate in the soil. They will emerge safe and sound when the soil temperature increases, in the spring.

In case garden plants are not available in that area, beetle larvae have the ability to feed on grass-roots as well. This amazing adaptability to feeding on all kinds of available plants and plant roots is what made them thrive and expand their territory throughout the world. In Japan, they are being kept under control by their natural predators.

But once they became invasive to other areas of the world, with no immediate natural predator and a high level of adaptability, they rapidly became a pest. The first record of this insect on United States territory dates as far back as 1916, in a nursery in the state of New Jersey. The first written evidence of its presence in Europe was in Milan, Italy.

How about adult Japanese beetles?

Adult beetles have the ability to fly over many miles of land, looking for the right area to feed. Once it landed on a proper plant, the beetle sends out pheromones, this way calling more beetles to join them in feeding and mating. Adult beetles only feed on leaves and will work their way from the top of the plant downwards.

When many beetles feed on the same plant at the same time, the plant becomes overwhelmed, and soon after, it dies. Since adult beetles only eat the leaf part in between the leaf’s veins, the plant soon becomes skeletonized, and it can no longer feed through photosynthesis. This pattern has a major impact on the economic loss of farmers.

Once both male and female beetles are feeding on a plant, mating begins, and females will soon after descend to the soil to lay their first eggs. Eggs are laid close to the soil’s surface. Adult beetles are most active towards the end of May all through the end of August. It is during this time that Japanese beetle traps will be most effective.


Japanese beetle management

Plants and grass that are healthy and well taken care of are by far less susceptible to being attacked by these insects than plants that are not. Keeping a close eye on your garden ensures you properly spot the first signs of a beetle invasion. If you begin physically removing them from the plants, you can keep a small infestation under control.

If, however, for any reason you miss out on the starting point of the infestation, and what you have before your eyes now is a mass of plants with bugs on them, then you need to choose one of the other available beetle management solutions. Setting out beetle traps could be one of them. Beetle traps have proved to be quite effective.

Japanese beetle traps are made of two parts: the top and the collecting bag. The top part is impregnated with pheromones and flower scents to attract the beetles, while the bottom part consists of a collecting bag. The bugs will fall into this bag and will never find their way out of it. There might be a disadvantage to using this management method.


More information on Japanese beetle traps

Chemical pheromones used to make this type of trap are stronger than natural pheromones that beetles send out while feeding, thus attracting many more beetles than the usual number in that area or garden. Some consider this to be a downside to using these traps. However, you would be doing your neighbors a favor.

A much more effective way of using these traps has proved to be getting the whole community involved in this process. Since about 70% of the adult beetle population is caught when using these traps, it will be a lot better to strategically set traps across the entire area, than to have a few isolated traps in a couple of neighboring yards.

Also, they should be placed as far away as possible from the plants you are trying to protect from the beetles. These traps are to be kept fresh and clean at all times, following the packaging instructions to ensure maximum effectiveness, and also you should start using them in the  beginning of June all through August when beetles are most active.

What to avoid when dealing with Japanese beetles

In case of a massive, out of control infestation of Japanese beetles in your garden, the advised thing to do is to use chemical pesticides, that will rapidly kill close to 100% of the existing adult beetles, preventing them from further feeding and reproducing before it is too late for your plants. This is a sure way to get rid of them fast.

If, however, the infestation level allows you to take other methods into consideration, then you should not use pesticides in your garden. Why is that? Well, first of all, chemical insecticides are not selective to beetles only, so they will also kill close to 100% of the good and necessary insects in your garden. It will be a small ecosystem breakdown to your garden.

Furthermore, some of the dead insects may arise the interest of hungry birds, which may become intoxicated if ingesting the pesticides. And this way, an unwanted snowball effect will take place, without you even knowing it, when all you wanted to do was to get rid of the unwanted Japanese beetles.

Taking all the effects of such an action into consideration is of utmost importance to our little corner of nature. Raising awareness is a must, even amongst our neighbors and friends, ensuring this way a safer environment for ourselves, our families and the animals living around us.




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