In gardens across Minnesota, the beetle battle begins

By Andrew Krueger, MPR News

While a lot of Minnesotans have been pestered by mosquitoes and ticks over the past couple of months, another annoying insect is just starting to make itself known in gardens and fields across a wide swath of the state.

Sporting metallic green and copper colors, Japanese beetles are starting their annual feast of roses, grapevines, fruit trees and any number of other plants, including soybeans and some other crops.

The invasive insects — first found in the U.S. in 1916, and first spotted in Minnesota in 1968 — spent the past winter underground as grubs, chewing on grass roots. Now they’re emerging as adult beetles and causing headaches above ground.

The beetles “skeletonize leaves by feeding on tissue between the major veins, giving them a lace-like appearance,” the University of Minnesota Extension reports. They also eviscerate blossoms and blooms on rose bushes and some other flowering plants.

July and August are the peak months for Japanese beetle activity — and the peak months for gardeners trying various methods to try to keep the insects in check.

Do they hurt the plants?

The U of M Extension says that while the beetles can cause a lot of unsightly damage to leaves and flowers, healthy plants can survive an infestation without long-term effects.

But young plants, or plants that are already stressed — as many are from drought conditions the past few years — can be less able to withstand the onslaught.

And whether healthy or not, if vegetable plants or fruit trees are targeted by the beetles, it can reduce their yield.

And once this summer’s beetles lay eggs underground, the grubs that hatch will start feeding on roots and can cause damage to lawns.

Where are the beetles found in Minnesota?

The Minnesota Department of Agriculture has created a map showing where the beetles have been found in the state, including areas where they’re abundant.

Parts of Minnesota with significant infestations include the Twin Cities as well as areas near Mankato, Owatonna, Albert Lea, Rochester and Winona.

The invasive insects have not yet been reported across much of southwest, northwest and northeast Minnesota.

Beyond Minnesota, the U.S. Department of Agriculture says the beetles are widespread in most states in the eastern U.S. They’re not yet reported in states west of the Rocky Mountains.

What can be done to stop them?

The University of Minnesota Extension outlines several options for controlling Japanese beetles.

For smaller gardens, one common and simple method is to manually remove or knock the beetles from plants, and put them into a bucket of soapy water to kill them. The beetles have some pretty effective defense mechanisms — dropping off a plant at the slightest tremble of a leaf or stem — so it can take some trial and error to figure out an effective removal technique.

One key is starting early, when the first beetles are seen, because beetle activity draws more of the insects to a given area.

Japanese beetle traps can be found in stores but U of M experts advise against them, saying they can attract more beetles to an area than they catch — possibly causing even more damage to nearby gardens. The Star Tribune reported earlier this year that two U of M students are working on an improved trap that could alleviate some of those concerns.

Insecticides are another option but carry the risk of harming other insects, including beneficial pollinators.

The U of M Extension reports that a wasp and a fly species that prey on the beetles or their grubs have been released in Minnesota — though “neither is very abundant and they have little impact on Japanese beetle populations.”

Another control option, over time, is to choose plants that the beetles don’t target. And there’s always the option of just living with the beetles and the damage they cause.

Do you have a favorite, proven method for dealing with Japanese beetles, that you’d be willing to share? Send an email with more information to akrueger@mpr.org; if we get enough submissions, we’ll compile those ideas in a follow-up post.

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