Brace for impact: Japanese beetles are coming!

By Donna Tatting, Chisago County Master Gardener

Yes, those pesky Japanese beetles are back!

I live on the southern fringe of Chisago County, and until this year, I hadn’t seen a Japanese or its close relative, the False Japanese Beetle, anywhere in my gardens. That was until a week ago when lo and behold, I found two of them chomping away on my petunias. I quickly handpicked them, and after one squish between my fingers, that was that…so far.

The very fact that in 22 years of living here, this was the first time I’ve seen them in my gardens, so I know now that I will not escape their invasion. It may not be bad so far, but it usually only gets worse.

The following article by University of Minnesota Extension entomologist Jeffrey Hahn will provide you with some help, and there are a number of other good resources available online at the University of MN website:


Jeffrey Hahn, assistant Extension entomologist

The moment many people have been dreading has arrived — Japanese beetles (JB) are starting emerge.

It is not necessary to panic as they are not out in force yet. A few individuals have been found early (normally JB is not out until the first week of July). However, you know the rest are not too far behind. In fact with the recent rains, we could be seeing large numbers within a week or less.

JB is a pest because the adults feed on the leaves and flowers of many plants while the grubs feed on the roots of turf grass. If you have seen JB grub damage in the past, July is a good time to treat your yard. Use a preventative insecticide, like imidacloprid, after you see adults flying, about late June or early July this year. By the time eggs are laid and grubs hatch, about two to three weeks, the insecticide will be taken up by the grass and the young grubs will be exposed to it.

As the grubs get older they are less affected by preventative insecticides. It is still possible to control them with a curative insecticide, such as trichlorfon (e.g. Dylox). You can effectively treat JB with a curative insecticide until about mid-August. Remember to only treat the grubs if you are experiencing problems in your lawn. It is not effective to treat grubs to reduce the number of adults that are seen in your garden. Adult beetles are good fliers and can easily fly into your yard from the surrounding neighborhood.

You have a variety of options for managing the adults, including handpicking, low impact products like Neem and pyrethrins containing PBO, and residual insecticides, like permethrin and carbaryl (Sevin).

Another option is the use of a systemic insecticide, like imidacloprid (various trade names) and dinotefuran (Safari). They are easy to apply and long lasting. They do not kill JB quickly, but they do cause them to stop feeding with death coming later.

One important drawback of these products is they are very toxic to bees. Avoid treating plants, like linden and roses, that are very attractive to bees. It doesn’t matter that the trees and shrubs are not flowering at the time of application as these insecticides will be active for a year.

Another important consideration is that it takes some time, especially for imidacloprid, for the tree to translocate the insecticide (three to four weeks for large trees). If you have plants that have been plagued by JB in the past, now would be a good time to treat them with a systemic so the insecticide can protect them before much damage is inflicted.

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