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Dealing with Japanese beetles

Jeffrey Hahn, Extension Entomologist

If you had problems last summer with Japanese beetles on your property, the odds are very good they will be pests again this year. In fact some people are already seeing them in high numbers. So the question is what can a gardener do?

Killing individual Japanese beetles is actually easy but the problem is what to do when there are a lot of them. There is no one guaranteed method for treating Japanese beetles. It is best to use as many different management steps as possible to improve your chances of keeping Japanese beetle feeding to tolerable levels.

Have you checked your garden recently for Japanese beetles?
  Photo: Jeff Hahn, University of Minnesota Extension
If you have a small garden, physical removal can be a good option, especially when only small numbers are present. However when Japanese beetles are numerous, it becomes impractical to handpick them every day. Another non-chemical option is to place a cheese-cloth, or similar type of fabric, around valued plants to exclude the beetles.

There are a few low impact insecticides available. Pyola is effective but this product is short-lived and repeat applications are necessary. Neem oil is also an option. It helps deter Japanese beetles but is less effective when large numbers are present. Both of these products can be toxic to pollinators so be sure to apply them when bees are not active.

Thinking of using traps to control Japanese
beetles? Save your money, they are not
effective.  Photo: Jeff Hahn, U of M Ext.
There are a variety of residual insecticides that you can use, like permethrin or carbaryl (Sevin). To protect bees, apply insecticides during late evening after bees are no longer active. The products should be dry by morning when bees become active again. If you are trying to protect a large tree, you may need to have it treated by a landscape or tree care company.

Deciding whether to treat trees and other plants will depend on how many of the leaves are still intact. If nearly all of them are already chewed up, then spraying does not help protect the plants. If there are still a lot unaffected leaves, then it is worth your while to treat them.

Keep in mind that trees that are healthy and mature are not going to be killed from just one year of feeding. Trees are quite resilient and can tolerate a lot of defoliation. If trees has been recently transplanted or are already under stress, it is a good idea to protect them from any additional damage.

What you don’t do is just as important as what you do try. Some people like the idea of using Japanese beetle traps to protect their plants. While these traps can capture an impressive number of beetles, the traps are attracting more beetles than they catch. Research has shown that when these traps are used in home gardens, damage is not reduced but in fact is likely to increase. Likewise, companion planting has been demonstrated to be ineffective in protecting plants from Japanese beetle feeding.

 For more information, see Japanese beetle management in Minnesota.
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