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Managing Japanese beetle feeding

Over the last week, Japanese beetles have made an unwelcome return to Minnesota gardens. These gleaming copper and green beetles have been in Minnesota for decades but have become a more prominent pest in the last decade. This year’s batch of adult beetles started to show up recently, and models indicate that we have only seen a small percentage of the total number of beetles we will be dealing with this season. Expect to keep seeing beetles through mid-August.

Japanese beetles can gather in large groups. Photo: Melissa Schreiner, Colorado State University, Bugwood.org.

What’s a gardener to do? Take a deep breath and read on.

Feeding is a nuisance but not often deadly

A key thing to remember about adult Japanese beetles is that while feeding is an eyesore, it typically does not cause long-term damage to otherwise healthy plants.

Mature shrubs and trees, if healthy, can tolerate a lot of damage without long-term consequences. While beetles may feed on rose flowers, if the plant is healthy, it will make it through the feeding.

If the beetles are feeding on plants you prune in mid- to late-summer, think of the beetles as doing the pruning for you.

Japanese beetles aggregating on this flower look gross but are unlikely to meaningfully harm the plant.

What can I do about Japanese beetles?

Learn to live with them.

The Japanese beetle life cycle. Illustration: Joel Floyd, USDA APHIS.

Want an all-natural, hands-off, pesticide-free way to manage Japanese beetles? Manage your own expectations and learn to live with them.

The plants in our garden are part of the natural world, and as such, are going to be fed on. While the Japanese beetle isn’t a native insect, it is in Minnesota and here to stay.

Japanese beetles have been established in other parts of the Great Lakes region for decades, and people still have green lawns, productive vegetables and beautiful flowers.

Beetle populations fluctuate year to year, driven by how successful the grubs are at finding turf to feed on, which is harder for them in years when July and August are dry.

Does this sound like any lawns you knew last year? The impact of the last three years of drought on Japanese beetles may be revealed in the next few weeks.

There are parasitoid wasps and flies that lay eggs in and on Japanese beetles that have been reported in Minnesota. But studies in other states have shown that the establishment of these beneficial insects is highly variable, so it may be a long time before they make a dent in Minnesota beetle populations.

In most situations, plants can power through the largely cosmetic Japanese beetle feeding. Changing your garden expectations will save you money, sprays and stress.

Want to actively manage them? Start yesterday (though now is also good)!

Feeding by one Japanese beetle attracts more Japanese beetles.

Peak Japanese beetle population typically occurs in late July. Ideally, start removing adults as soon as they appear. A feeding Japanese beetle puts out smells that attract other Japanese beetles, so a few beetles feeding in an area can quickly become dozens.

The earlier you start working on getting beetle numbers and feeding down, the easier the rest of your summer will be.

Want to manage beetles organically? Roll up your sleeves.

Physically remove beetles—though a jar might not be a big enough vessel in all gardens.

Physically removing beetles is highly effective and the most practical option in many gardens. While walking through the garden (especially in the morning or evening), pluck beetles off plants as you see them. A bucket of soapy water can be a good companion on these walks, as beetles can be flicked into them. 

Busy? Let the neighborhood kids skip the lemonade stand and pay them a nickel per beetle they pick.

There can be a temptation to try to let a trap do the physical removal for you, but in general, traps tend to do more harm than good. In research studies, traps have been found to actually bring more beetles to an area. While some beetles end up in the trap, many end up outside of it — mating, feeding and laying eggs.

Research traps in Minnesota that monitor Japanese beetle populations have caught more than 5,000 beetles a day. If that is how many ended up in the trap, imagine how many of these clumsy beetles ended up nearby.

Thinking about spraying? Think hard before picking up the bottle. 

Check to make sure a pesticide is safe for pollinators.

Short on time, long on beetles? There are pesticides that can work, but they need to be used correctly. 

The hardest part of picking out a pesticide may be selecting something from a wall of products at the hardware store. However, the real challenge in using pesticides is choosing the right product and applying it according to label directions. Make sure the product can be used on Japanese beetle adults and on the plant you want to treat. 

Some parts of Minnesota will see some hot weather in the coming weeks, make sure to read the label of your product to see how weather impacts it. In hot weather, there is an increased risk of products drifting and moving into areas where they aren't meant to be used.

Reading the little booklet affixed to a spray bottle is not optional: it is the law! The label on a pesticide will tell you how to keep yourself, your family and your plants safe while using the product, as well as how to use the product effectively.

Homemade remedies may not kill adult beetles and can potentially hurt plants and non-target insects. Mixing up something from your pantry might not be safe or effective.

For more information on Japanese beetle control, including insecticide options, see Japanese beetles in yards and gardens.

Author: Marissa Schuh, Extension educator, horticulture integrated pest management

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