Japanese beetles vs. Wisconsin gardeners: As you wage war against the despised, invasive pests, here's what to know to get the upper hand (2022)

Japanese beetles vs. Wisconsin gardeners: As you wage war against the despised, invasive pests, here's what to know to get the upper hand (1)

Raise your hand if you’ve ever cringed in disgust at a glob of Japanese beetles in your garden or secretly delighted in sending them to their death.

What’s that, you can’t raise your hands, because you’re too busy using them to pick the destructive beetles off yourraspberries and roses? We hear you (yes, even the words we can’t repeat).

Japanese beetles are the uninvited guests who show up just in time for the Fourth of July backyard cookout, help themselves to whatever looks tasty and then don’t leave until sometime after Labor Day.

In other words, a pest — or, more appropriately, a capital P-E-S-T.

The metallic green and copper-colored beetles, native to Japan but invasivein the United States, are in prime feasting mode in Wisconsin in July and August. They feed on hundreds of plants, eating the soft tissue between the leaves' veins until all that’s left is a lacy skeleton and an unhappy gardener. They’re a source of what sometimes feels like endless misery for those who grow.

To help in the battle of the bad beetle, we called in an expert.

Entomologist PJ Liesch is director of the University of Wisconsin-Madison Insect Diagnostic Lab, aka @WiBugGuy on Twitter. He’s been studying the not-so-little buggers for nearly 15 years and graciously agreed to share his insights, offer some tips and bust a few myths.

First, a quick review of the Japanese beetle life cycle, so we know what we’re dealing with here.

The adults you see now are feeding, mating and laying eggs in turf grass. Once those eggs hatch, the larvae or white grubs, feed on the roots of grass through the rest of summer. In fall, they move down 8 to 12 inches below the surface to overwinter. In spring, as the soil warms, the grubs move back up, feed for a bit, pupate in late spring and early summer and then emerge as adults.

“So one generation per year,” Liesch said.

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When is Japanese beetleseason?

You’re looking at it. They generally begin to emerge in large numbers the first week of July and then begin to drop off as we get into September. That doesn’t mean Liesch hasn’t seen some as early as mid-June or as late as early October.

“From a plant injury perspective, I think of, for us in Wisconsin, the months of July and August as our main periods of concern,” he said.

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Are some years worse than others or does it just feel like they're all bad?

Weather can be a factor. A brutally cold winter, especially one with little or no snow cover as insulation, can affect the survival rate of grubs underground, which means fewer hungry adults to emerge the next summer. This year, Liesch is seeing numbers down across the state. He attributes that to last year’s dry conditions.

There’s nowhere females love more to lay their eggs than a lush, green lawn. Many lawns went dormant during last summer’s drought, which made them less favorable as an egg-laying site. If females did lay eggs in a lawn but it wasn’t watered, conditions may have gotten too dry for the young larvae to survive.

It’s also not uncommon for certain areas in the state to have bigger numbers and more damage than others. The same saying that applies to gardening also applies to Japanese beetles: “No two years are the same,” Liesch said.

What plants are among their favorites to feed on?

Roses are right up there at the top of the list. The most prized of flowers, so that figures, right? They also quite enjoy grape vines, pole beans, raspberries, basil, linden and basswood trees, certain birches, crabapples, fruit trees like apple and plum, and sweet corn (they can clip the silks off and disrupt pollination).

Keep in mind they feed on the leaves and flowers of more than 350 species of fruits, vegetables, ornamentals, field crops and weeds, so theirs is an extensive menu.

Do they actually kill the plants?

It depends on the plant, but for the most part, they’re not outright murderers. They can sure wreck the appearance of something like that tropical canna you planted to enjoy on your deck though.

Larger plants, like trees and shrubs, put out enough leaves that they can tolerate the damage. Even plants that get completely skeletonized by Japanese beetles one summer are likely to leaf out fine the next, Liesch said. But if a plant keeps getting damaged by beetles year after year, like some birches,it can weaken or stress that plant, making it susceptible to other diseases or issues.

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A common method forcontrolling them is to pick or knock them off into a jar of soapy water. Is it effective?

It certainly can be, especially for gardeners with small yards who have the time to diligently stay on top of it. During the COVID-19 lockdown in 2020, as many people suddenly found themselves working from home, Liesch frequently heard how gardeners would take a computer break during the day and go knock some beetles off. That’s not as much of an option now with more people back at the office.

Research has found it can help, especially when it’s done early in the season. Here’s why: As Japanese beetles start to damage a plant, volatiles from the plant are released in the air. Other beetles smell those chemicals and are drawn to the plant. It's theequivalent of a “Hey guys, over here!”

“It’s kind of like blood in the water drawing in sharks,” Liesch said.

Despite popular belief, Japanese beetles do not have an aggregation pheromone.

“The idea is that one would start feeding, send out a chemical signal and attract their friends,” Liesch said. “That is not true. They don’t have an aggregation pheromone.”

For that reason, you can dump your stinky jar or bucket of dead Japanese beetlesin a corner of your yard where you dispose of yard waste without worrying it will attract more.

What about squishing them? Is it true that will only attract more?

Another myth. Females have a sex pheromone, “a chemical perfume that she releases to attract males for mating purposes,” Liesch said. That’s where the myth comes from that if you squish a female, the pheromone is going to attract more.

Instead, what happens is once she mates with a male, which is almost right away after emerging from the ground, she is no longer producing that sex pheromone.

“So you can go and squish as many Japanese beetles as you want and you really don’t have that risk of drawing more into the area,” he said.

Do they bite?


“I can’t completely write it off as out of the realm of possibility. I have held hundreds of them from doing research over the years. I can’t say I’ve ever been bit by them,” Liesch said.

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Because of their names, Liesch said people often confuse Japanese beetles with Asian lady beetles. The latter are the ones that sneak into the house in the fall, and they have been known to “nip” on occasion.

To maximize our picking/dunking/squishing efforts, does the weather impact how many are out and feeding on any given day?

Just like most of us, they’re fond of warm, sunny weather. They tend to be most active on pleasant summer days in the 70s and 80s. They can be less active when temperatures get extremely hot or on a cooler, rainy days in the 60s.

Are those Japanese beetle traps yousee hanging in people’s yards a good idea?

Pheromone traps do attract and collect beetles.

“However, do they actually make a difference in your yard in terms of reducing plant damage? Several studies have looked at that and found that they do not help,” Liesch said. “What happens is you draw more beetles to the general area. You’re going to collect some of those but many more are simply landing on nearby plants and feeding.”

Research has found that when using traps, nearby plants tend to see more damage.

“We generally do not recommend using those traps because at best you’re probably breaking even. But in most cases, you’re probably actually bringing on more damage and causing more harm than doing good,” he said.

Are there other control options?

Fine mesh netting with holes small enough that beetles can’t get through isn’t ideal for situations like a large tree, flowers that you want to look nice or crops like raspberries that need pollinators to have access, but it could be an option for something like covering your pot of basil.

There are some sprays available, both organic and conventional, but many of them can kill pollinators, including bees, which most gardeners are trying to attract.

What about trying to control them as grubs?

Liesch gets that question a lot. Targeting the beetles as larvae or white grubs in your lawn won’t have much impact on the number of adult beetles you’ll see next year, as more will just fly in from other areas. Grub control can make sense at placeslike a golf course, where there’s low tolerance for turf damage. But most homeowners are going to have some grubs, and most robust lawns can tolerate it without damage to the grass.

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You can help out the cause by not watering your grass from July through mid-August.Green lawns are like a flashing “vacancy” sign for females to lay their eggs.

How long have they been in Wisconsin anyway and when did they get so annoying?

Japanese beetles were first discovered in the United States in 1916 in New Jersey.The first reports of them in Wisconsin go back 50 years or more, Liesch said, but it’s more of "a recent phenomenon” of the last 20-plus years that they have become abundant and problematic in parts of the state.

They are not everywhere in Wisconsin. If you were to draw a line from Green Bay to Wausau to Eau Claire, they are fairly widely distributed south of that line. North of it, however, they are more patchy. There have been sightings in the last decade in Oneida and Vilas counties, and more recently in Bayfield County and Marquette, Michigan.

“So they’re starting to push their way northward. It wouldn’t surprise me if over time with changing climate we started seeing them more in northern Wisconsin,” Liesch said.

He’s also observed that as the beetles push into a new area, their damage can be quite significant at first but then tends to become less so over time.

“I almost think of it kind of like a wildfire, where when that fire first hits an area it’s very hot and damaging and then after a while you’re left with more of slower smolder.”

Part of that might be natural enemies, like predators and parasites, helping to find a balance.

So do Japanese beetles have any redeeming qualities?

“Yeah, that’s a good question,” Liesch said.

There are some birds and mammals that will eat them, either as adults or larvae below ground, so that’s at least something.

Japanese beetles vs. Wisconsin gardeners: As you wage war against the despised, invasive pests, here's what to know to get the upper hand (4)

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ContactKendra Meinertat 920-431-8347 or kmeinert@greenbay.gannett.com. Follow her on Twitter @KendraMeinert.

This article originally appeared on Green Bay Press-Gazette: What Wisconsin gardeners should know about Japanese beetles


Are Japanese beetles invasive in Wisconsin? ›

Japanese beetles are common garden pests in Wisconsin. The Japanese beetle (Popillia japonica) population in Wisconsin typically peaks in mid-summer. This invasive insect is a defoliating species that devours multiple landscape and garden plants.

How invasive are Japanese beetles? ›

The Japanese beetle is a highly destructive plant pest that can be very difficult and expensive to control. Feeding on grass roots, Japanese beetle grubs damage lawns, golf courses, and pastures. Japanese beetle adults attack the foliage, flowers, or fruits of more than 300 different ornamental and agricultural plants.

When did Japanese beetles come to Wisconsin? ›

The two-banded Japanese weevil was first spotted in Wisconsin in 2016. Move over Popillia japonica, there's a new "Japanese beetle" in town. Wisconsin typically sees a few new invasive species each year.

What do Japanese beetles do? ›

Japanese beetles are destructive plant pests in both their immature and mature forms. As larvae, the insects chew on grass roots, creating large dead spots in turf. Adult beetles gather in large numbers on garden vegetation, eating the leaves and petals until they take on a skeletal appearance.

What kills Japanese beetles on contact? ›

Sevin Insect Killer Ready To Spray, designed for hose-end spraying, makes treating trees and shrubs simple. This powerful pesticide kills Japanese beetles and more than 500 other insect pests by contact, then it keeps protecting your plants, blooms and lawn for up to three months.

What trees are Japanese beetles attracted to? ›

Favorite plants include roses, crabapples, pin oak, hibiscus, grapes, raspberries, linden, crape myrtle, sassafras, Japanese maple and Norway maple.

What is a natural way to get rid of Japanese beetles? ›

A simple solution of water and dish soap can suffocate Japanese beetles. Grab a bucket and mix a quart of water with a teaspoon of dish soap. Once you mix the water with dish soap, the least “touchy” solution is to pour the soapy water into a spray bottle and spray the beetles on your affected plants.

What attracts Japanese beetles to your yard? ›

Various flower, fruit, and plant fragrances will lure in these pests, particularly trees of the black walnut, cherry, apple, and linden family. They also have quite the sweet tooth for grapes, plums, roses, and hollyhocks.

How do I get rid of Japanese beetles in my house? ›

Though simple, a mixture of soap and water is pretty effective at eliminating Japanese beetles. In a spray bottle, mix a quart of water with a teaspoon of dish soap, and spray any plants that appear to be infested.

Can you squish Japanese beetles? ›

It is OKAY to squish adult beetles on the spot; their remains (pheromones) do not attract more beetles to the plants to feed.

Do Japanese beetles bite? ›

While Japanese beetles do have robust mandibles (teeth) they use to chew leaves, their teeth are too weak to break through skin and they do not bite people.

Do Japanese beetles harm pine trees? ›

Because pine trees have needles rather than leaves, adult Japanese beetles do little damage to pine trees. The most damage done by these insects is actually perpetrated by the grubs laid in the soil at the base of the tree. Shortly after emerging, adult Japanese beetles lay their eggs in the soil beneath trees.

What is eating my four o clocks? ›

You are right: Japanese beetles love to dine on four o'clocks, and according to several university sources these plants are poisonous to them. They are also toxic to people and pets. They can cause vomiting and diarrhea if eaten and the sap can cause dermatitis.

Does neem oil keep Japanese beetles away? ›

Neem oil works best on small-scale infestations but can be an effective weapon against Japanese beetles and their grubs.

What kills Japanese beetles but not bees? ›

Neem Oil: We also deter feeding by adult beetles by spraying plants with Neem oil. Neem oil and sprays containing potassium bicarbonate are somewhat effective, especially on roses. The adult beetles ingest a chemical in the neem oil and pass it on in their eggs, and the resulting larvae die before they become adults.

How do you get rid of Japanese beetles in Wisconsin? ›

A simple solution of a squirt or two of dish washing soap and water provides an effective trap for Japanese beetles. A simple solution of a squirt or two of dish washing soap and water provides an effective trap for Japanese beetles.

Are there Japanese beetles in Wisconsin? ›

As July arrives in Wisconsin, Japanese beetles become active and can cause problems for gardeners, landscapers, and farmers. Activity often peaks in late July and August.

How long do Japanese beetles last in Wisconsin? ›

Japanese Beetles have made a comeback in Wisconsin this year, emerging from the ground in mid-June. They are present for about six to eight weeks every summer. Each beetle lives from 30 to 45 days. Fortunately, most Japanese Beetles are gone by mid-to late August.

Does squishing Japanese beetles attract more? ›

It is thought that crushing them releases a pheromone that attracts more beetles. Research has found that crushing does not draw more feeders. It is true that Japanese beetles are drawn to an area by the use of pheromones. Japanese beetle traps, or lures, are baited with the scent to attract the adults.


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