Arboretum News

Japanese Beetles: They’re Back!

Our integrated pest management specialist shares some tips for deterring them.

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Japanese beetles causing damage to a bloom on the Rose Walk at the Arboretum. Photo by Susie Eaton Hopper.

By Erin Buchholz, integrated pest management specialist

We get so many questions about Japanese beetles! Here’s what I recommend to help keep people and wildlife safe:

Go out in the early morning to handpick them off of your plants and toss them into a bucket of soapy water. You can use dish soap or laundry detergent – just enough to create a little bit of suds. Once there, they won’t be able to fly out. But you need to let them sit in it for a day, or they will not die. Early morning is usually cool enough to keep them lazy so they won’t fly around. By doing this, you won’t have as many adults laying eggs in your soil, which means less of them next year.

We let our large trees go. There is no practical way to spray or drench trees with insecticide that will not also adversely affect beneficial insects. If they are established, the beetles will not harm them permanently.

Yes, it’s an eyesore. My paper birch has had more than half of its leaves skeletonized before, and their droppings underneath it are disgusting. But my tree lives, and it is not their preferred meal every year. They love lindens (also known as basswood), so if you are choosing a new tree and Japanese beetles have been an issue for you in the past, try another species.

I don’t recommend lures, as research has shown that they attract more beetles to your yard, and only 20 percent of those drawn in will actually go into the trap. We only use them to monitor numbers and compare to past years’ data.

Japanese beetles on a table grape vine in the Home Demonstration Garden at the Arboretum. Photo by Erin Buchholz.

Do we treat grubs? No! The Arboretum is in agreement with University Extension and Master Gardeners that grub treatment is expensive and not as effective as it should be. First, milky spore is not meant for northern climates – too cold! Plus, it takes years to establish itself in soils in warmer climates.

Nematodes can be effective, but you need to make sure they are alive when you receive them in the mail. A lot of times, they die in transit. Unless you have a microscope, you won’t know they were dead until you realize they didn’t work at all.

Pesticides for turf are often systemic ones that can harm bees. If you have clover and other plants in a bee lawn, “bee” careful! Please research which chemicals you would use, as ones strong enough to kill the beetles are also strong enough to harm our beneficial insects (as well as birds and fish).

If you can tolerate a tan lawn, ease back on your watering! Starting in July, ease back a little and see if it helps, as irrigation can make your yard more attractive.

Some Facebook commenters have also used fine mesh or netting on their prized plants with great success. That’s not the best option for us at the Arboretum as we have so many plants needing protection, but I think it’s a great mechanical control for small yards and gardens! Keep in mind, it could also prevent pollinators from reaching your crop.

Wild turkeys love to eat Japanese beetles, as do backyard chickens. Encourage your feathered friends, if you are comfortable doing so. It’s free protein, and we love to watch our turkeys jump three feet off the ground to gobble them up.

For more information, please visit UMN Extension – Japanese beetles. Or, leave a comment if your questions aren’t answered there. Good luck!

5 comments on “Japanese Beetles: They’re Back!

  1. Pingback: Arb Links, vol. 23 | News from the Minnesota Landscape Arboretum

  2. Robert Alsleben

    Here in Waconia e have the Japanese Beetles on the grape leaves, honey crisp apple leaves, plumb leaves, rasberry & pole bean leaves. Last year we used the attracting traps and you are correct that just brought more. Last Fall used the Biotech’s Shrub and Tree Protect and Feed around the base supposedly to kill the larve and they still came but a little later, so maybe from other places. Isn’t there something that will make them impotent because they are continuously breeding.

    • Hi Robert,
      Researchers in Europe are currently working on pheromone lures to draw males away from female breeding grounds with other insect pests (moths). I would hope that could be researched on JBs, too. We have also used a product called Organocide (not the 3 in 1, but the fish/sesame oil one), and saw that their feeding was reduced. I’m not sure if less feeding would lead to less propagation, but fingers are always crossed.
      As for the product you used, if the active ingredient is imidacloprid, please know that that is a neonicotinoid, so it will harm pollinators if it gets into flowering weeds, trees and shrubs, as it is systemic – travels all throughout the plant’s systems.
      As I see more and more birds feasting on them, my hope is our native predator populations will start to help keep balance. They are still relatively new to our region, so it takes time.
      Japanese beetles don’t really kill a plant. Repeated leaf loss can stress a plant out eventually leading to death, but they change their feeding habits up. Sadly, it’s one pest I am trying to be more tolerant of, as I love other wildlife more than I hate them.
      Best,
      Erin

  3. What about Neem oil? Used every few days it seems to work and is supposed to be safe for pollinators.

    • Hi Peg,
      Neem should only be applied when pollinators are not active. Best time to apply is just before sundown, as long as temperatures are below 85 degrees F (to avoid burning leaves). Making direct contact with neem is bad for any insect, including beneficials. It breaks down more quickly than other insecticides, so it’s considered a better choice for some. There are some that use it as a systemic insecticide, as they drench it in the soil. However, I don’t think enough research has been done to show its efficacy, or safety to pollinators via that route.
      We use neem here, too, but we are super careful to time the applications to protect our desirable critters.
      Best,
      Erin

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