Gardening News: Controlling the invasive Japanese beetle

Japanese beetles are invading gardens, flower beds, and crop fields across northern Illinois. Insecticides, parasites, and pathogenic bacteria are used to control them. (Photo by Dom Castaldo)

By Dom Castaldo, Ph.D.

A couple weeks ago, I wandered over to our arbor to check on the progress of our grapes. I was horrified to find the once-lush vines covered with Japanese beetles. They were devouring the grape leaves. I mixed up a batch of carbaryl (Sevin) — a mild insecticide — and sprayed it on my unwanted dinner guests.

Within a few minutes, it was raining — Japanese beetles. With every dead beetle that hit the sidewalk, my smile grew a bit wider. The next morning, I was sporting a very broad grin. At least 100 dead Japanese beetles were on the walk and unknown number of carcasses in the grass. Since my initial assault, I have not observed any more Japanese beetles on the grape vines. Our grapes are safe.

I should mention that after I sprayed our grapes, I noticed that a second army of Japanese beetles were munching on our rose bushes. I sprayed them, too.
Many people are unaware of the history or biology of the Japanese beetle. Japanese beetles are not native to North America. They are an invasive species that arrived in New Jersey in 1916. It is unclear if Japanese beetles were deliberately imported as pets — their iridescent metallic-green wings are attractive — or if they were stowaways on cargo from Japan. Since their arrival, they have spread across the eastern U.S. and Midwest — including northern Illinois — wreaking havoc and destruction in flower and vegetable gardens and crop fields.

After mating in middle to late summer, adult female Japanese beetles lay approximately 50 eggs in the soil. The eggs develop into white larva. These are the grubs homeowners find in the soil as they prepare their gardens in the spring. During early to middle July in northern Illinois, adult Japanese beetles emerge from the soil and begin chewing on plants in preparation for another cycle.

Japanese beetles are not fussy eaters. They have been called “a stomach with wings.” Adult Japanese beetles eat more than 300 species of plants in northern Illinois — including grapes and roses. They will consume the leaves, flowers and other parts of a plant. Japanese beetles eat the soft green part of leaves, leaving a “skeleton” of leaf veins. In severe beetle infestations, Japanese beetles can substantially reduce plant and crop yields.

Some reports suggest Japanese beetle populations follow explosion-crash lifecycle patterns. Each year, numbers of the beetles in a geographical area increase (explosion) until predators or disease causes large numbers of the beetles to die (crash).

One reason non-native species, such as Japanese beetles, thrive in their new homes is because they have few natural predators. However, Japanese beetles do have some predators in northern Illinois. Starlings, robins, catbirds, blue jays and cardinals eat both adult beetles and Japanese beetle grubs. Other birds, such as grackles, meadowlarks, crows and blackbirds, only eat grubs. Ducks, chickens and guinea fowl eat large numbers of adult Japanese beetles.

In addition, skunks, raccoons and moles will dig and consume grubs they find in lawns. Some spiders and ants consume large numbers of Japanese beetle eggs.
Spring tiphia wasps have been imported from China to control Japanese beetles. The wasp larva parasitizes, and eventually kills Japanese beetle grubs.

At the microbiological level, some homeowners use milky spore to keep Japanese beetle populations — and the damage they cause — low. Milky spore is a pathogenic disease of Japanese beetle. It is caused by Paenibacillus popilliae. When the bacteria is added to the soil, it infects and kills the Japanese beetle grubs. The bacteria do not harm humans, pets or plants. Although milky spore is effective, it may take three to five years to reduce or eliminate the grubs from the soil.

Chemically, Sevin — carbaryl — is the most effective insecticide. It easily kills adult Japanese beetles, but does not harm humans or pets when used in recommended concentrations. Permethrin, deltamethrin and spinosad are other insecticides that are effective against Japanese beetles. In the early fall, homeowners should apply imidacloprid, a granular insecticide, to their lawns and flower beds to kill the Japanese beetle larvae living in the soil.

Some homeowners prefer to knock Japanese beetles from leaves and into a soap solution. The soap removes the buoyancy from the insects, and they drown.
Most insect experts do not recommend using traps to control Japanese beetles. Most traps contain pheromones or flower aroma to attract adult beetles. The beetles enter the trap and die. Although traps can kill many Japanese beetles, researchers reported that the traps attracted more Japanese beetles to an area than they kill.

Meantime, I’ll keep patrolling our vegetation for signs of the tiny invader.

Dr. Dom Castaldo lives in Mt. Morris. He is a biology instructor at Sauk Valley Community College in Dixon, and can be contacted via e-mail at

From the Aug. 3-9, 2011, issue

One thought on “Gardening News: Controlling the invasive Japanese beetle

  • Aug 6, 2014 at 1:35 pm

    Most enlightening! There was a male cardinal on my pole beans when I drove up, and his mate on my fence vine on the way into the house. I was hoping maybe they were having a lunch date!

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