Japanese beetles, otherwise known as Popillia japonica, are one of the most significant pests in North America. They are native to Japan, where a number of natural predators keep the species in check. Since being introduced by accident in 1916 in a New Jersey nursery, the Japanese beetle has spread nearly everywhere in North America, attacking upwards of 200 species of plants, including rose bushes, grapes, hops, canna lilies, crepe myrtles and others.

Japanese beetles tend to eat leaves and flowers, leaving the parts that are too tough for their jaws. In this case, they eat the soft leaf parts but leave the fibrous veins, producing a characteristic "web" look.

Fortunately, although they can do a lot of damage, you can get rid of Japanese beetles. Here's why they always show up in June and how to tackle them.

What do Japanese Beetles Look Like?

Adult Japanese beetles are about a half- inch long, with a shiny metallic green body and legs. The wing covers, or elytra, are coppery-brown; the thorax is shiny green. White stripes on the lower end of the abdomen give them the appearance of small bees, which protects them from predators; they also buzz when they fly, further enhancing the illusion.

With their long legs and shiny colors, they are distinctive and actually quite attractive. If they weren't such a destructive pest, these beetles might be welcome in your garden as a pretty little addition to the bees and butterflies flying around your flowers.

How Did Japanese Beetles Get to Be Such a Pest?

In places where this beetle is a native, its population is controlled by natural predators. In North America, however, P. japonica is an invasive species. This means there are few to none of the natural predators that keep its numbers in check in Japan. Without as many predators, the Japanese beetle has been able to flourish and spread throughout much of North America. Although there are some natural control methods (see below), there are not enough of these countermeasures in the natural environment of the US to keep them under control.

Why Do They Emerge in June?

Beetles undergo complete metamorphosis, which means they have four stages of development. In this way they are much like butterflies, wasps and flies. Japanese beetles, therefore, have an egg, larva, pupa and adult stage. The adult is of course the beetle that we see on our roses and other plants. The other stages are generally subterranean and are seldom seen until they turn into the beetles and emerge.

June is when most broods of the Japanese beetles emerge. The larva, or grub, lives and feeds underground. Although it can do some damage to the roots of grasses and other plants, you likely will not know they’re there (occasionally you may dig one up; they look like curled-up little caterpillars and are milky white with a brown head). In June, these larvae emerge from their pupal stage and the adult beetles fly off to find your rose bushes and start chewing on the flowers and leaves.

Controlling the Japanese Beetle 

The simplest way to control japanese beetles on your plants is to take some time and pick them off by hand. A pair of garden gloves and a bucket of soapy water is all you need, and it is surprisingly effective; you can also  take care of a bunch at once by putting a bucket of soapy water under the plant and giving it a shake. Other insects like sawfly grubs can also be dealt with this way. 

You may also try netting your roses, but Japanese beetles are quick and persistent and will eventually find a way in under or through the netting. Fortunately, there are other, less labor-intensive control methods that you can use. Among the most popular are a biologic substance that attacks the grubs and passive traps that attract and kill the adults.

1. Biologics and Passive Traps

One popular, non-chemical control method is to apply a bacterium that attacks only Japanese beetles. This pathogen causes a disease (“milky spore disease”) that is deadly to Japanese beetles but harmless to the beneficial insects in your garden. Although it sounds like something out of a zombie movie, this method is considered quite popular and safe. Milky spore is a naturally occurring host-specific bacterium (Bacillus popillae Dutky), that acts as a natural control vector for P. japonica. You apply the bacterium to the ground around your plants, where it comes into contact with the grubs, killing them.

There are also passive attractant traps that draw adult Japanese beetles into a container using pheromone bait. Since the adult beetles are the only stage that mate and reproduce, they are vulnerable to control methods that use artificial pheremones as lures. You never have to handle or even see the dead beetles; just dispose of the container and set out a new one. This method is used by professional biologists and entomologists to attract and assess pest insect populations.