Japanese beetles (Popillia japonica) were not much of a problem in the United States until about 1919, when this ravenous native of Japan began a serious invasion, probably after hitchhiking to North American on imported ornamental plants. Today, they are a serious nuisance to gardeners and farmers across North America. The adult Japanese beetles that eat so many plants are about 3/8-inch long, with a shiny, metallic green body and copper-brown wing covers. However, not all metallic green or copper beetles are Japanese beetles. To make sure you're dealing with Japanese beetles, check their undersides for five small, white tufts under the wing covers and an additional tuft at the end of the abdomen.
Why Japanese Beetles Are a Problem
While individual Japanese beetles aren't a particular threat, these insects tend to hatch in very large numbers, and when they congregate on a shrub, tree, or other plants, they can quickly defoliate it. They tend to run in cycles; some years are almost devoid of beetles, while in other years they are present everywhere. When there is an infestation, it is often a very large one that can seriously damage large numbers of plants. The sheer numbers make it hard to eliminate them, but there are still steps you can take to control and prevent Japanese beetles.
There’s a wide smorgasbord of plants for Japanese beetles to feast on—more than 300 varieties of trees, shrubs, and non-woody plants. During a year with a serious infestation, gardeners may feel as though there is no plant that the beetles don't enjoy.
Understanding the Japanese Beetle Lifecycle
Beetles typically go through four stages of development. It’s important to know when they are passing through each stage in your climate because control methods are different for each stage. The life stages for the Japanese beetle are:
- Egg: The mature beetles lay small, oval, white eggs in the soil. If moisture is sufficient, the eggs will absorb it and enlarge, becoming rounder as they do.
- Larvae: This is the white grub stage that is so familiar (and so damaging) in lawns. Japanese beetle larvae have a V-shaped series of bristles on their raster (the underside of the tip of the abdomen). Grubs will grow in length as they feed and mature.
- Pupae: The pupae stage is where the grub starts to transform into a beetle. Japanese beetle pupae start as cream-colored and age to a reddish-brown.
- Adult beetle: The adult beetles are less than 1/2-inch long, and the shell is shiny, metallic green. Adults emerge from the ground between May and June, depending on your area. They live for about 30 to 50 days, during which they feed and reproduce.
When the first beetles emerge from the ground, they immediately look for suitable plants and start feeding. They also send out an odor known as a congregation pheromone to signal later emerging beetles where to go. Mating starts soon after.
The female beetle feed on plants for a couple of days, then burrow into the soil to lay their eggs. Shortly after, they will return to feeding and mating and start the cycle all over again. By the end of the season, each female Japanese beetle will have laid about 50 eggs.
Eggs develop at different rates in different soil temperatures, developing most rapidly in warm soils of about 80 to 90 degrees Fahrenheit. When soil conditions temperatures are ideal for eggs to hatch, you can expect an infestation of lawn grubs to follow shortly, with large numbers of beetles appearing in the following year.
Once the eggs develop into larvae, they will move up toward the surface of the soil and start feeding on roots and organic matter. In large numbers, these grubs can cause severe damage to a lawn, sometimes killing off large areas. This is why grub control is usually applied in late summer to fall. The pesticide needs to be applied while the grubs are feeding on the grass roots.
As the soil cools and the grubs mature, they move back down deeper into the soil for winter. They’ll stay there until the soil warms in the spring, at which time they burrow back up toward the surface where they’ll pupate and eventually emerge as adults. The cycle now begins all over.
When to Control Japanese Beetles
The proper timing for controlling Japanese beetles depends on what stage of the beetle's lifecycle you are trying to attack.
- Hand-picking or spraying with chemical or natural pesticides should be done while the beetles are actively feeding on plants, a period that lasts for roughly a month in late May, June, or July.
- Attacking the larvae stage (the grubs) is normally done in late summer to fall when the grubs are maturing and moving upward to feed on roots.
It’s impossible to get rid of Japanese beetles entirely. More will fly in as the current crop are killed. And there aren’t many natural controls for adult Japanese beetles. Birds aren’t partial to them, and although some predatory wasps and flies have been imported, their population isn’t large enough yet to control the Japanese beetle problem. So controlling this pest will require you to use one or several hands-on methods. Keep in mind that the adult Japanese beetles are only around for a little over a month, so don’t automatically reach for harsh chemicals unless they become a serious problem.
Removing Beetles by Hand
The most effective natural control is to remove beetles from your plants by hand. Some gardeners crush the beetles between their fingers, but most people brush the beetles off the plants and into a jar of water mixed with dish soap, where the beetles quickly drown. Japanese beetles generally feed in groups starting at the tops of plants, so it's usually pretty easy to fill a jar with them. But you will need to do this every few days for the month or so that the beetle infestation lasts.
Fighting Beetles With Insecticidal Soap
Insecticidal soap will kill adult Japanese beetles only if it is sprayed directly on the beetle. It does not have any residual effect, meaning that beetles that aren’t sprayed directly won’t be harmed. However, insecticidal soap may also kill other helpful insects, such as ladybugs.
Any variety of synthetic pesticides will kill Japanese beetles, but to be effective they need to be sprayed in fairly large volume and will simultaneously kill other insects, as well as pollute the surroundings.
A much better choice is to use neem oil or a pesticide containing pyrethrin—a biodegradable substance derived from chrysanthemums. Pyrethrin insecticide should still be applied selectively, directly on the beetles, since it will also kill other insects. But this substance has no residual effect on the environment and is thus one of the safest pesticides you can use.
Controlling Lawn Grubs With Chemicals
A long-term approach to controlling Japanese beetles is to attack them at the source—in the ground where the beetle larvae (grubs) feed and mature. This is normally done with a granular "grubicide" applied to lawns in late summer or fall when the grubs are moving up into the root zone to feed. Grub baits are somewhat controversial since they are synthetic chemicals, but most are classified as only "mildly toxic" by the EPA. With careful application following label directions exactly, this is a relatively safe chemical pesticide. Grub baits are fairly selective pesticides that affect a variety of lawn-damaging beetles and grubs but have less impact on earthworms and other beneficial organisms. Avoid any product that contains Sevin, as this pesticide does kill earthworms and other beneficial organisms.
If you have repeated intense infestations of Japanese beetles, check your soil in late summer to see if you have a large grub population. Lift a 1-square-foot section of turf. If there are more than a dozen grubs in this small area, consider treating your lawn with some type of grub control. Remember that not every garden with a Japanese beetle problem has a lawn full of grubs. The beetles can hatch in your neighbor's lawn and find your tasty garden with very little effort.
Combating Beetles With Milky Spores and Nematodes
Two natural organisms can be very effective at controlling the larvae of Japanese beetles, although it can take several years to fully enjoy the benefits.
Milky spores are a natural live organism (Bacillus popilliae) that is applied to the lawn. Once grubs become infected, they die within seven to 21 days, and the organisms multiply and spread through the soil as the dead grubs decompose. Once established in the soil, milky spores can protect against Japanese beetles for more than 20 years. But it can take as long as five years in colder climates for the spores to become thoroughly established.
Nematodes are tiny microscopic worms found in the soil. Some types are harmful to plants, but the types commonly used for Japanese beetle control (Steinernema spp.) are ones that target beetle grubs. Nematodes can be killed by direct sunlight, so apply them on a cloudy day or as night is falling. Water them in, since this will assist their passage into the soil.
Both milky spores and nematodes can be applied at the same time. They make a very effective long-term solution to lawn grub and Japanese beetle problems.
A word of caution about the pheromone beetle traps. They attract beetles; you’ll probably wind up with more beetles in your yard than before. The original intention of the traps was to track when and how many Japanese beetles were in the area, not as a means of eradication.
Tips for Controlling Japanese Beetles
The presence of Japanese beetles can also be accompanied by the presence of moles ruining your lawn. The reason is that the moles have arrived to feed on the grubs under the ground. Controlling lawn grubs can therefore not only eliminate Japanese beetles but also get rid of your mole problem.
Where Japanese beetles are an ongoing problem, you may want to consider planting your landscape with plants that are less attractive to them. Some plants to consider:
- Red maple
Japanese Beetles in Yards and Gardens. University of Minnesota Extension.
Japanese Beetles. West Virginia Extension Service.
Organic Management Options for the Japanese Beetle at Home Gardens. Integrated Pest Management University of Missouri.
Cranshaw, W. Japanese Beetle - 5.601. Colorado State University Extension.