One of the most frustrating things that happens to gardeners is putting in the effort to grow beautiful, healthy plants, only to have them chewed up by pests. You may have noticed that some of the leaves on your plants look skeletal – that is, most of the leaf tissue has been eaten, leaving behind the veins and petiole. This is likely the work of Japanese beetles. Japanese beetles (Popillia japonica) are a particularly persistent invasive species that’s become a major pest in the garden. Even government-run control programs have failed to fully suppress populations, so gardeners should be prepared to keep this pest out of the garden.
This article will review various control methods to see which ones are best.
Why Are Japanese Beetles a Problem?
Japanese beetles (JB) are a major issue because they are generalists – they eat hundreds of different plants . They see most gardens and farms as an all-you-can-eat buffet, with lots of choice.
The beetles lay eggs slightly below the soil surface, which then hatch into larvae that devour plant roots. As the larvae get bigger they consume bigger and bigger roots, which can damage or kill plants. The larvae typically eat the roots of grasses since they have many fibrous roots near the soil surface. A lot of turf and pasture area has been decimated by Japanese beetle larvae.
After the larvae pupate and develop into adult beetles, they fly around, up to 5 miles (8 kilometers) at a time, and eat the foliage of almost any plant they find. What’s worse is that plants produce chemicals as they are being eaten and these chemicals attract even more beetles. The beetles usually avoid thick, tough foliage and woody tissue. They prefer to eat the soft tissue between leaf veins or they take out large chunks from flowers.
As their name suggests, Japanese beetles are native to Japan and came over to North America in the early 1900s. They are also now found in some parts of Europe. Since these beetles are non-native, there are no native predators that suppress them, so plant growers and gardeners need to take matters into their own hands.
Japanese beetles are easy to identify – they are oval and metallic green, with shiny bronze wing coverings. The white dots at the back are quite unique.
Drowning Beetles in Soapy Water
While drowning beetles sounds macabre, it’s a very effective way to control Japanese beetles. You simply fill a container with water, add some dish soap, then flick beetles into the container, where the soapy water kills them fairly quickly. This method is a cheap and eco-friendly way to control pests since you don’t need to apply any substances to the plants or soil. It works best early in the morning when the beetles are more lethargic.
The dead beetles can be added to a compost pile, or just dumped on the ground where birds and other critters will eat them.
Since the soapy water method is pretty time intensive, I wouldn’t recommend it to people who are busy or have a large property. It’s best used if you happen to see a bunch of beetles in one area of your garden and want to quickly get rid of them before they damage plants. Repeat the process on a daily basis until they are all gone. This method won’t get rid of any larvae in the soil but it does reduce the number of eggs laid this year.
Japanese Beetle Traps
Beetle traps are one of the most popular options for controlling Japanese beetles because they require almost no effort. All you have to do is buy the trap, hang it up and throw it away when it gets full. The trap is usually a special bag or container that the beetles can enter but not escape They are attracted to the trap through scented chemicals that smell like dead flowers and/or female sex pheromones.
The problem with the trap is that they attract beetles from the whole neighborhood, not just your garden. You might collect a lot of beetles, but the number in your yard, eating your plants, stays high. To use the trap properly it should be placed at least 100 feet (30 m) away from the plants you are protecting. This works great in a large yard, but not in smaller home gardens.
When Japanese beetle traps are used in areas with a lot of beetles, spillover of the traps can become a major issue. This happens when the collection bags get too full and excess beetles are no longer trapped. In this case, you end up with more beetles in your yard then you would have without a trap.
Neem Oil for Japanese Beetles
Neem Oil is a popular biological pesticide that is sometimes recommended for Japanese beetles. You can find out more about Neem oil in Neem Oil Insecticide and Fungicide for Plants.
Even though it is recommended a lot on social media, the science shows it is not very effective. Neem can work in three different ways; as a suffocating oil, an antifeedant (stops feeding) or as a poison. As pointed out in the above link there are two basic types of neem and you need to be very sure you are using the right one or it won’t work at all.
Neem works well on several kinds of small insects because the oil covers the insect and suffocates them. It is great for things like aphids and scale, but JB are too large and an oil coating is not effective.
Neem oil can be an effective antifeedant, but studies have shown that Japanese beetles soon get used to the taste and it loses its potency. It also has to be applied every few days to work.
As a pesticide, neem contains azadirachtin, which interrupts adult molting and affects egg laying. One study looking at roses, used Monterey Neem Oil and found that it only killed 5% of the beetles while conventional pesticides had a mortality rate in the 80 to 90% range.
Products that contain pyrethrins are somewhat effective as a contact insecticide. They have to be sprayed right on the beetle and spraying needs to be repeated as new beetles arrive. Avoid spraying bees and other beneficial insects as these products are toxic to them as well.
Is this easier than using soapy water?
Trap Crops to Control Japanese Beetles
Trap crops are sacrificial plants that are more delicious to the pest in question than the main crop, keeping pests off of them.
Japanese beetles feed on over 300 different plant species, but they prefer plants like Oenothera (evening primrose), Zinnia, Pelargonium (annual geraniums), Borago (borage) and Vitis (grape vines). If you plant these near your garden, then lots of beetles will feed on the trap crop instead of your garden plants.
When the beetles gather on the trap crops you can spray them with a fast knock-down insecticide or collect and drown them in soapy water.
Controlling JB Grubs
All of the above methods target the beetle, but you can get some control by killing the grubs before they hatch. The best control for this are beneficial nematodes. They are applied to lawn areas where they are soaked into the soil. “These nematodes have a mutualistic symbiotic relationship with a single species of bacteria. Upon penetrating a grub, the nematode inoculates it with the bacteria. The bacteria reproduce quickly, feeding on the grub tissue. The nematode then feeds on this bacteria and progresses through its own life cycle, reproducing and ultimately killing the grub.”
Remember that nematodes are alive. They should be purchased fresh and used as soon as you make up the diluted solution. Follow all directs on the package, including the date for application. Nematodes are effective if the right ones are used (Heterorhabditis bacteriophora), they are applied properly, and at the right time of year.
But here is the problem. Killing all the grubs in your lawn does not mean you will be beetle-free the following year. When beetles hatch in other yards or wild areas they can travel quite far to find your delicious plants. Nematodes really only work if all your neighbors also use them.
Written by: Marika Li