Ideas for controlling that pesky Japanese Beetle

Environmental Notes  
AJ Moses
Oakdale Environmental Management Commission    

Let’s admit it – part of our environmental awareness is a result of the aesthetic beauty of our landscapes. Full, lush and green we like. Shriveled, defoliated and brown we do not. Enter the Japanese beetle – JB for short.

This pest existed only in Japan until about 100 years ago when it was accidentally introduced in New Jersey.

Why are these beetles such a problem? According to the University of Minnesota Extension, JBs forage on over 300 species of plants. They have a few favorites – and a few plants they won’t touch. Ones they like include linden trees, roses, vining plants (such as grapes and pole beans), and, well, the list goes on. It’s reported they don’t like buckthorn or poison ivy, but who plants these in their landscape, anyway?

And good luck controlling this pest. First, they fly great distances to aggregate on favored plants. Because plants give off odors that attract JBs, and then JBs give off additional odors that attract more of their kind, keeping them away is difficult. Spraying is only marginally effective. Unless a potent, broad spectrum pesticide hits the insect directly, chances are good spraying won’t work. Also, the conundrum we face is that broad spectrum pesticides kill beneficial insects, too.

Systemic insecticides, especially imidacloprid, should not be used on plants that attract bees, such as linden trees and roses because they are very harmful to these useful insects. Further, these insecticides are so persistent that, when applied in early spring, they can kill pollinators months later when plants and treated trees flower. Trace amounts have been found in fruits of trees treated in spring, which not only points to their effectiveness, but also means people who eat the fruit are also ingesting the chemicals. (However, many of these insecticides are new enough that we don’t know the long-term impact of people ingesting these chemicals.) Topical sprays are different from systemic pesticides, but both can be absorbed to some extent by the fruit. 

If you can control the grubs, you will reduce your turf damage, as the larva feed on the roots of your grass. Knowing the life-cycle of the Japanese Beetle is helpful to this endeavor. JBs emerge from the turf in late June or early July as adults. They tend to aggregate near the tops of plants to feed and mate during the day. As the temperature cools in the evening they fly to the turf where the females lay their eggs. (Each female can lay up to 60 eggs.) Next step – the grubs start hatching by mid-July and feed on the roots of your turf. This cycle continues through mid to late August. The grubs continue to feed on your grass roots through September. 

Once the soil temperature cools, they tend to burrow deeper into the soil where they remain for the winter. In early May, the mature grubs – which are nearly an inch long – migrate back near the surface and continue munching on your turf roots. By early June they pupate, and the cycle of adult emergence begins again.

Now that you know their lifecycle, it’s good to know when they are vulnerable. The most effective time to treat your turf is mid-June through late mid-August. This is when the grubs are most vulnerable. The grubs emerging in May are too robust to succumb to pesticides applied to turf, and the pupae and adults are not impacted either.

There are signs that will indicate you have a JB infestation in your turf. A brown, nearly circular patch is a visual key. If you grab a handful of browned turf and pull and it rolls up like a loose carpet, this is a sign that the roots were eaten away by JB grubs. (Note, there are other pests that eat turf roots, too, so it’s important to continue your examination.) You probably won’t see any grubs under this loosened turf. They likely have already moved to the nice, green turf surrounding the dead patch. Look there. To help identify the grub, you can see photos and specific details on the University of Minnesota Extension Website at http://www1.extension.umn.edu/garden/insects/find/japanese-beetles/. 

So, what about those traps? According to the article above, more beetles are attracted to the vicinity of these traps – your landscape – than are caught. The result is a higher concentration of JBs in your yard! What about those broad-spectrum pesticides to kill adult JBs? One of the most commonly used – and most available to home-owners – is carbaryl, branded as Sevin. Note – this product is highly toxic to bees, earthworms and birds. It’s also toxic to fish and amphibians. Carbaryl should not be used adjacent to wetlands. Bifenthrin and other pyrethroids are highly toxic to honeybees, fish and birds. Remember, these products are effective only for about two weeks after application.

So, what to do about those adult JBs once they invade? One option is to don a pair of gloves, put a gallon of water and a tablespoon of dish soap in a bucket and start picking the JBs off your plants. Once in the soapy water they cannot fly out. Other long-term solutions include introducing plant species they don’t like. The U of M article has a list of the most vulnerable commonly planted species. They also include a list of plants seldom damaged. 

I strongly urge you to do some additional homework before you take any course of action to control this – or any landscape pest. If you choose a pesticide, please follow label instructions carefully – and wear protective equipment.

Thanks to Jeffrey Hahn of the University of Minnesota Extension Master Gardener Program for reviewing this article!

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