The city gardener

Let's talk about Japanese beetles


It’s July, and that means it’s Japanese beetle season in Minnesota!
Japanese beetles have been a pest in the United States since they were accidentally introduced in the early 1900s. Over 100 years later, we still battle this invasive species each summer. I’ve spent many hours picking the beetles from my grape vines and currant bushes, and I’m sure many of you have, too. You may be wondering if there’s a better way to control this common pest.
The good news is that Japanese beetle damage is usually only cosmetic. Although bug-eaten leaves are not very attractive, it usually will not kill a mature plant. However, Japanese beetle grubs can occasionally damage lawns. If a dead patch of grass can be rolled back like a carpet, its roots may have been eaten by Japanese beetle grubs.
Early July is a good time to start thinking about Japanese beetle control. Japanese beetle grubs spend the winter underground, where they mature into adults in the spring. Adult beetles emerge from the ground in late June or early July and feed for 6 to 8 weeks. That means they are usually done feeding by late August.
Start looking around your yard for adult beetles now. They are approximately a half inch long and are metallic green and bronze in color. You are most likely to find them in groups, on plants that are in direct sunlight.
It’s best to control Japanese beetles as soon as you start to see them because once leaves are damaged by beetles, they emit odors that attract even more beetles. So the more damage you have, the more you will get! On the other hand, it also means that Japanese beetles are attracted to individual plants that are emitting the odor, so you may see large numbers of beetles on one plant while nearby plants have very few beetles. You may choose to “sacrifice” a plant to the beetles in hopes that they are attracted only to that plant and leave other plants alone. You could even try placing a plant that the beetles particularly like (such as roses or hollyhocks) in a far corner of your yard, to draw them away from your favorite plants.
There are a number of different options for controlling Japanese beetles, which may be more or less practical depending on how much space you have. In a smaller yard, you can simply remove adult beetles from plants by hand and drop them into a bucket of soapy water to kill them. The beetles move more slowly when it is cool out, so it may be easier to catch them in the morning or evening.
If you have a specific plant that you want to protect, try covering it with fine netting to prevent beetles from reaching it. If the plant needs to be pollinated (like fruit trees), hand pick beetles until the plant is done blooming and then cover with netting. You don’t want to prevent pollinators from reaching the blooms.
There are some insecticides that can be used to control Japanese beetles, but be careful to avoid harming bees and other beneficial pollinators. Check the label to make sure the product is approved for Japanese beetle control and follow the label instructions closely. However, Japanese beetles rarely cause serious harm, so consider avoiding pesticides if possible.
Japanese beetle traps are widely advertised, but they are not effective for controlling beetles in your yard. Traps use synthetic pheromones to attract beetles, but this often just makes the problem worse by attracting even more beetles than would otherwise have been present. University research has demonstrated that a lot of beetles fly towards the traps, but not all of them are caught. Rather, many beetles end up on nearby plants, where they cause damage.
Finally, when choosing new plants for your yard, consider planting species that Japanese beetles tend to avoid, including clematis, daylily, geranium, boxwood, and magnolia.
For more information, check out the University of Minnesota Extension Yard and Garden website. Extension resources are written by experts and contain the latest and most reliable research-based information. Happy gardening!
Lauren Bethke is a Hennepin County Master Gardener Intern with a passion for homegrown vegetables, pollinators, and everything green. She lives in Hiawatha with her husband and pets.


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