Voracious Japanese Beetles

These little critters are a gardener’s worst nightmare, devouring more than 300 varieties of plants.

Voracious Japanese Beetles

Voracious Japanese Beetles

Japanese beetle. Image courtesy of R.J. Reynolds Tobacco Company Slide Set, R.J. Reynolds Tobacco Company, Bugwood.org.

Japanese beetle. Image courtesy of R.J. Reynolds Tobacco Company Slide Set, R.J. Reynolds Tobacco Company, Bugwood.org.

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An unwelcome import from Japan, the Japanese beetle was first found in North America in 1916.  Roughly 3/8” long with coppery green coloring, these leaf eaters are found across much of the United States, but are most common east of the Mississippi. In adult stages, these beetles are aggressive and indiscriminate pests, consuming the foliage or flowers of over 300 varieties of ornamental and agricultural plants. Damage to trees and plants from these easily identified pests can be extreme, but problems begin elsewhere while the insect is still in larval form.

Louise Romanow, an entomologist and master gardener trained at North Carolina State University explains. “Japanese beetles are a problem with many kinds of foliage; they can be even more of a problem for people with their lawns while they’re still in the grub stage. Basically, they just eat away at the root systems of the grass and they leave terrible brown patches all over the place from the damage.”

Many commercial treatments of these grubs can lead to other issues explains Romanow.  “The chemicals people tend to put down to deal with the problem are indiscriminate, so they end up killing everything. They kill the bad beetles, but they also kill the good beetles. There are lots of predaceous (pest eating) beetles in there too.”

Beneficial nematodes can be introduced to soil to address these grubs. Another strategy lies with milky spores, a fungus specific to Japanese beetles. This organic treatment is effective, but can take several years to establish. Even then, long-term balance is rarely achieved. Spores build up, wipe out a beetle population and then die off, at which point beetles will return until spores have time to build up again. “You never really get rid of them,” says Romanow, “but it is a good, organic way to  manage Japanese beetles, although it takes a long time and most people want to see that bug dead right away.”

Adult Japanese beetles, most prevalent in the hottest summer months, require different management techniques. Adult beetles release pheromones to draw other beetles to feeding sites, so quick action is necessary to prevent a growing infestation.

During those summer months, hand-picking beetles as they appear is an effective and non-invasive control method, but does require diligence. When numbers are still low, scouts can be easily removed from plants in the morning or late evening when beetles are least active and crushed or drowned in soapy water. Daily inspection of plants is advised.

Although commercially available, the use of Japanese beetle traps is discouraged. Baited with beetle attracting scents, these traps are likely to draw more beetles to the site than the traps will accommodate.

Chemical treatments are effective in the management of Japanese beetles. Select insecticides with a narrow spectrum to minimize collateral damage to beneficial insects. Residual effects of an application earlier in the season will generally last through the season without continuing treatment.

In areas where Japanese beetles are a problem, choosing the right plants will circumvent problems without invasive tactics. Boxwood, daisies, begonias, magnolias and hydrangeas are all non-attractants and will encourage these destructive pests to seek out a different location.

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