Controlling Japanese Beetles

There is hardly a gardener out there who hasn’t encountered a Japanese beetle. The adult Japanese beetle is a shiny, metallic green with copper-brown wing covers and it’s about 3/8-inch in length. Not all metallic green or copper beetles are Japanese Beetles. To be sure of what you’re dealing with, you can look closely at the underside of the beetle and you’ll also see 5 small, white tufts under the wing covers and an additional tuft at the end of the abdomen.

Japanese beetles can create havoc in a garden by feeding on the leaves of a number of different plants, skeletonizing the leaves and eventually defoliating the plants. An individual Japanese beetle doesn’t do that much damage while feeding on a plant, but they tend to congregate in large numbers and can easily defoliate shrubs and trees. Keep in mind that the adult Japanese beetles are only around for a little over a month, so don’t automatically reach for the sprayer unless they become a serious problem. Some years are better than others.

The first beetles to emerge start to look for suitable plants to eat and start feeding immediately. They also send out an odor known as a congregation pheromone to signal later emerging beetles where to go. Mating starts soon after.

The females will feed on your plants for a couple of days and 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 – 90 degrees F. Once they develop into larvae, they will move up toward the surface of the soil and start feeding on roots and organic matter. 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 start moving 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.

It’s impossible to get rid of Japanese beetles entirely. More will fly in as the current crop are killed.

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.

The most effective natural control is to go into your garden with a jar of soapy water and knock the beetles into it. Japanese beetles feed in groups, starting at the top of plants, so it’s actually pretty easy to fill a jar with them.

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.
A word of caution about the pheromone beetle traps. They will attract beetles, but you’ll probably wind up with more beetles coming into your yard than you would have without the trap. 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.

Finally, if you have repeated intense infestations, you should check your soil in late summer to see if you have a large grub population. Lift a 1 sq. ft. 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. However, not every garden that has a Japanese beetle problem is associated with a lawn full of grubs. The beetles can hatch in your neighbors lawn and find your tasty garden with very little effort.

There are several insecticides labeled for use on adult Japanese beetles, including:

Bayer Advanced Garden Multi-Insect Killer Concentrate
Carbaryl (Sevin)
Neem extracts
Ortho Bug-B-Gon Garden & Landscape Insect Killer Concentrate
Orthene Turf, Tree & Ornamental Insecticide
Spectracide® Bug Stop Multi-Purpose Insect Control Concentrate
Spectracide® Triazicide® Soil & Turf Insect Killer Concentrate

These sprays will kill on contact and also have some residual effect. Keep in mind that these sprays will kill more than just the Japanese beetles, so use them only for extreme infestations. And again, Japanese beetles are only a pest for a little over a month, so don’t over react.
Always read and follow label directions when using any pesticide.