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Japanese beetles continue to pose threat to gardens

Gloria Hafemeister
Correspondent
Japanese beetles pose problems for gardeners, especially on berries and roses.  A recent on-line workshop featured advice from “Wisconsin Bug Guy” P.J. Liesch of the UW-Madison Diagnostic Lab.

MADISON – Japanese beetles continue to pose threats to gardeners in Eastern and Midwestern parts of the United States, including southern Wisconsin.  Recently they have also begun migrating to northern parts of the state and have been spotted as far north as Bayfield County.

The iridescent green and copper colored beetles do not discriminate when it comes to the types of plants they feed on.  In fact they like about 300 different common plants but they particularly favor roses and berries.

Patrick (PJ) Liesch "the Wisconsin Bug Guy" is the director of the UW-Madison Insect Diagnostic Lab. He recently hosted an on-line workshop to help gardeners identify and control the pests. Nearly 900 people participated in the conference.

He says prior to the beetle’s accidental introduction to the United States in the early 1900s, the Japanese beetle was found only on the islands of Japan, isolated by water and kept in check by its natural predators. In 1912, a law was passed that made it illegal to import plants rooted in soil. Unfortunately, the failure to implement the law immediately allowed the Japanese beetle to arrive in this country.

They lay eggs in the soil during June, which develop into tiny white grubs with brown heads and six legs that are up to ¾ inch in length. These grubs will remain underground for about 10 months, overwintering and growing in the soil.

He says, “As temperatures get colder they move down so only extremely harsh winters will kill them but only in soil that is not protected.  If the ground is covered with snow, the soil does not get as cold and they can survive.”

Japanese beetles can skeletonize leaves like raspberries.

They emerge from the soil as adult beetles and begin feeding in early July. They usually attack plants in groups, which is why damage is so severe. Although the life-cycle of the adult Japanese beetle is barely 40 days, it can cover a lot of ground.

He notes, “These beetles love lawns. When they lay their eggs it is a batch here and a batch there. If your rose bushes or raspberries are next to grass that’s an ideal location for them.”

Japanese beetles can devour most of the foliage on favored plants. They nibble around the veins of leaves giving the leaves a skeletonized look. Japanese beetles are not usually far from damaged leaves, so inspect the plant thoroughly. Also keep an eye on the ground beneath the plant; the beetles may reflexively drop off the plant if disturbed.

Beetles are particularly attracted to apples, stone fruit, raspberries, grapes, strawberries, beans, corn, asparagus and herbs. While the damage in corn may not be immediately visible, he says they eat on the silk and this prevents pollination and the development of kernels.

“Injured plants will particularly attract them because they release a volatile chemical that the beetles like,” he notes.

He also notes that the grubs in lawns can create brown spots because they eat on the roots. A healthy lawn can tolerate some grub damage so mowing higher and proper fertilization will help. While watering makes a lawn look lush in summer, a dormant lawn is unattractive to the beetles.

So if they are present, how do gardeners get rid of them?

Liesch says what works one year may not work the next. Also, there are many commercial products out there that do not perform as well as their ads claim. Likely no one method will get rid of all of them.

“If you have the time you can knock them off by hand into soapy water. Do this at peak feeding times on key plants,” he suggests. “The earlier you start the practice the better.”

He also suggests choosing plants that are less attractive to the beetles but that may be difficult because they like more than 300 different plants.

Row covers during the peak feeding weeks will help but they also interfere with pollination because they keep pollinators such as bees off, too. Covering flowers also defeats the purpose of raising flowers to beautify the yard. 

“Japanese beetle traps can be helpful in controlling large numbers of beetles, but they also might attract beetles from beyond your yard,” he suggests. “Unfortunately, the traps do not effectively suppress adults and might even result in a higher localized population.”

If traps are used they need to be far enough away from favored plants or they will do more harm than good.   

Some participants in the conference wondered if there are any natural predators or parasites that help.

“Yes, ants in the soil and birds, including chickens, will eat them,” he replied. “Skunks, starlings and crows eat grubs but they themselves can cause problems to lawns.”

There are some chemical products that will kill them but caution needs to be used. Growers need to carefully read the labels to observe which plants are safe to use them not and which ones are not. Some treat grubs but do not eliminate beetle. Some are harmful to bees and pollinators.  Some are toxins on food crops.

When using spray or dust with insecticides, speak to the local cooperative extension office or garden center about approved insecticides in the area.

When spraying, missed spots and new growth will not be protected and beetles will still come.  Insecticidal soap works on contact but does not eliminate them. Rain will also wash off products. 

Homemade sprays can run more of a risk of damaging plant leaves, so be careful and use sparingly.

Systemic products will be helpful but they must be used early in season to be effective.

On trees, he recommends seeking the help of a professional. Some trees can handle some damage, he said.

“Trees can tolerate up to 30% leaf loss without overall health and yield affected. When spraying trees it is important to read the restriction labels,” he cautions.

There are federal laws prohibiting the spraying of some trees such as linden.