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What to do about Japanese beetles

Jeffrey Hahn, Extension Entomologist

Japanese beetles have taken a lot of people by surprise by their abundance this summer, especially in the Twin Cities. They were numerous in the early 2010’s but a series of very dry summers reduced their populations. Japanese beetle eggs and recently hatched grubs do not survive very well in dry soils. However, we have had more normal precipitation the last few years which has allowed Japanese beetle numbers to rebound. Japanese beetles also started emerging sooner than expected this year as they were first sighted in late June. As a consequence, a lot of people are faced with damaged plants and questions about what they should do now.

One common question people ask is whether their tree will die because of Japanese beetle feeding. If the tree is healthy and mature, the tree is not going to die in one year from Japanese beetle feeding. Trees are quite resilient and can tolerate a lot of defoliation.

It is actually easy to kill a Japanese beetle but it is when they are so numerous that it is challenging to manage them so their damage is at acceptable levels. Physical removal is a good option, although it is a labor intensive method; plus if you are trying to protect a tree you will only be able to reach so high to get them. However, this is the best non-chemical option.
Although this linden does not appear to be to
be healthy, it will not be killed by Japanese
beetle feeding and will recover.  Photo:
Jeff Hahn, UMN Extension

Another non-chemical method that people consider using is Japanese beetle traps. While they can capture an impressive number of beetles, it is not likely to protect your garden. In fact, research has shown that when these traps are used in home gardens, you are more likely to see more damage as the traps attract more Japanese beetles than they can capture.

There are a few low impact insecticides available. Pyrethrins are effective but this product has no residual and beetles have to be hit directly by the spray. It also needs to be reapplied more than once. Neem oil is also an option. It helps to 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.

There are residual insecticides that you can use, like permethrin or carbaryl (Sevin). To protect bees, you need to apply insecticides during late evening after bees are not active. The products should dry by morning when the 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 much of the leaves is still intact. If nearly all of it is already chewed up, then spraying does not help them. If there are still a lot leaves that are not affected yet, then it is worth your while to go ahead treat.

Some people wonder whether they should treat the grubs in their lawns to reduce the adults. However, this is not effective. The adults are very mobile and can easily fly onto your property from adjacent areas. Only treat your turf if you are seeing turf damage. The best time to treat preventatively is about the time Japanese beetles are laying eggs which late June or early July. You can treat curatively into August. There are a variety of products that can be used. Insecticides containing chlorantraniliprole (e.g. GrubEx) can be used both curatively and preventatively.
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