Starting in June, Japanese beetles have made an unwelcome return to Minnesota gardens. These gleaming copper and green beetles have been in Minnesota for decades but became a more prominent invasive pest in some parts of the state in 2011.
This year’s batch of adult beetles started to show up a few weeks ago. Populations will continue rising through July and taper off by mid-August.
Feeding is a nuisance, but not deadly
A key thing to remember about adult Japanese beetles is that while feeding is an eye-sore, it typically does not cause long-term damage to 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, the plant will make it through the feeding.
If beetles are feeding on plants you prune in mid to late-summer, think of the beetles as doing the pruning for you!
Note that this is true for healthy plants. Drought-stressed plants in your yard and garden may require more Japanese beetle control efforts than in the past.
What can I do about Japanese beetles?
The lowest-input method? Learn to live with them.
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 from year to year, driven by how successful the grubs are at finding turf to feed on. This is harder for them in years when July and August are dry.
There are parasitoid wasps and flies that lay eggs inside Japanese beetles that have been reported anecdotally 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, time and stress.
Want to actively manage them? Start yesterday (though now is also good)!
The Japanese beetle population typically peaks in late July.
Ideally, start removing adults as soon as they appear. This is because damaged leaves let off scent signals that Japanese beetles are able to detect, attracting more beetles to the damaged plants and surrounding plants.
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 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 of 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 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 meant to 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 using insecticide.
It may seem like the hardest part of picking out a pesticide is 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.
It may be annoying to peel open the little booklet affixed to a spray bottle, but it is not optional — it is the LAW!
The label of a pesticide is going to tell you how to keep yourself, your family, and your plants safe while using the product, as well as how to use the product in an effective way.
While mixing up something from your pantry means there isn’t a label to read, that doesn’t mean what you make will be safe or effective. Homemade remedies may not kill adult beetles and can potentially hurt plants and non-target insects.
For more information on Japanese beetle control, including insecticide options, see Japanese beetles in yards and gardens.
Cappaert, D. L., & Smitley, D. R. (2002). Parasitoids and pathogens of Japanese beetle (Coleoptera: Scarabaeidae) in southern Michigan. Environmental Entomology, 31(3), 573-580.
Carter Gordon, F., & Potter, D. A. (1985). Efficiency of Japanese beetle (Coleoptera: Scarabaeidae) traps in reducing defoliation of plants in the urban landscape and effect on larval density in turf. Journal of economic entomology, 78(4), 774-778.
Carter F. Gordon, Daniel A. Potter, Japanese Beetle (Coleoptera: Scarabaeidae) Traps: Evaluation of Single and Multiple Arrangements for Reducing Defoliation in Urban Landscape, Journal of Economic Entomology, Volume 79, Issue 5, 1 October 1986, Pages 1381–1384, https://doi.org/10.1093/jee/79.5.1381
McDonald, R., Puttler, B., Klein, M., Oliver, J., Grundler, J., Brown, M. E., ... & Burfitt, C. (2020). Establishment of Tiphia vernalis (Hymenoptera: Tiphiidae), a Naturalized Parasitoid of the Japanese Beetle, Popillia japonica (Coleoptera: Scarabaeidae), in Meramec State Park, Sullivan, Missouri, USA. Journal of Entomological Science, 55(1), 130-136.
Switzer, P. V., Enstrom, P. C., & Schoenick, C. A. (2009). Behavioral explanations underlying the lack of trap effectiveness for small-scale management of Japanese beetles (Coleoptera: Scarabaeidae). Journal of economic entomology, 102(3), 934-940.