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Extension > Yard and Garden News > Japanese beetles: Aftermath

Tuesday, September 26, 2017

Japanese beetles: Aftermath

Jeffrey Hahn, Extension Entomologist

Now that we are safely past the bump in the road called Japanese beetles (JB), it is time to take stock in what happened this year, whether we should be doing anything now, and even what to expect for 2018. Clearly, 2017 was a year of above average numbers of JB. However, this is not the first time we had high numbers of JB in Minnesota and it is unlikely to be the last. So why did we have so many Japanese beetles this year?
Japanese beetles were bad this year.  However, most plants
will survive this damage just fine.  Photo: Jeff Hahn,
University of Minnesota Extension

Although it might appear that JB occur in cycles (like forest tent caterpillars), in fact their numbers are driven by weather. The most important factor influencing their abundance is a lack of soil moisture. JB eggs and newly emerged grubs are very susceptible to dry soils so during years when we experience drought we typically see relatively fewer JB.

This was true about three to five years ago when we had some very dry summers. However starting in about 2014/2015 we started receiving more normal rainfall and JB numbers responded by becoming more numerous.

There has been a lot of concern by residents as to whether JB feeding would kill plants, especially trees.  While JB feeding definitely affects the appearance of plants, as long as (deciduous) trees and shrubs are healthy and mature, they can tolerate severe, even complete, defoliation.  However, if trees and shrubs are severely damaged for several consecutive years, they can sustain more lasting injury.  Maintain normal care for trees and shrubs to keep up their health.  Herbaceous plants are more vulnerable to JB feeding but also are able to tolerate some damage.  Look for most plants to rebound and be fine next spring.

Many residents also wanted to know about treating grubs to reduce the number of JB numbers on a person’s property. While it makes some sense to try to kill the grubs in your lawn so the adults don’t emerge and find your plants, the reality is that treating the grubs has no impact on how many adults you will find on your property next summer. The reason is that adult JB are good fliers and can fly from as far away as several miles.

If you have plants that are attractive to JBs, the adults will find them regardless of whether you treat for grubs or not. Only treat your turf if grubs are found damaging the grass. This is best done in July and August and into early September. By fall, the grubs are getting too large to effectively treat. They are definitely too large to treat the following spring.

So what about next year? We received pretty regular rains in 2017 throughout areas where JB is common. That strongly suggests that if you saw a lot of JB this year, you most likely will see at least similar numbers next summer. Stay tuned!

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