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Some Questions About Japanese Beetles

David Cappaert, Michigan State University,

Photo 1: Japanese beetle grub

Jeffrey Hahn, Asst. Extension Entomologist

There have been a lot of questions concerning Japanese beetles (JB) as we go into the 2012 growing season.  JB has been increasing in numbers over the last four or five years, especially in the Twin Cities area, although JB are also starting to be found more commonly in other areas of the state as well.  They can be challenging to control and people will take any steps they can to reduce their numbers in their gardens and yards.

The first question people ask is how bad are JB going to be this year.  There isn't an easy answer to that question and it undoubtedly will vary according to where you live.  If JB was abundant last year, there is a good chance they will be common again this summer.  However, a factor that can have an impact on JB numbers is soil moisture.   The eggs and the young grubs have a harder time surviving in dry soil so if dry conditions exist when JB are laying eggs, that can reduce the numbers of adults that are seen the following year.  Of course, if the turf area is well watered, that will make it easier for JB to survive.

It is difficult to determine what role the winter weather had on JB populations.  Although the mild temperatures favored JB, the lack of snow cover could have been potentially detrimental to them.  The early spring we have been experiencing should not have any effect on JB numbers.  However, expect them to emerge earlier than normal.  In a typical season, JB emerge around the 4th of July.  If the weather holds, the adults could be active as soon as the 3rd or 4th week of June.

And speaking of the early spring, people are wondering whether they can still treat JB grubs this spring or whether it is too late.  An important to factor to first consider is whether your primary goal is to control the grubs, because you are seeing damage in your turf, or the adults because of damage they have done to garden and landscape plants.  If your aim is to reduce the adults by controlling the grubs, you can save yourself the effort as this is not effective.  The adults are mobile and can easily fly in from areas outside your property. 

If you are finding turf damage due to JB grubs, and if you have had a lot of adults on your property look carefully for this, then the best time to treat for grubs is in July.  If you are going to use a preventative, such as imidacloprid (e.g. Merit), then you should be treating your turf when you first see the adults flying.  About two to three weeks after the first adults have emerged, JB eggs are hatching.  Preventative insecticides are most effective against the first instar larvae but not the older second and third instar larvae so the timing is critical. 

If JB grubs are not treated then, it is still possible to control them with a curative insecticide, such as trichlorfon (e.g. Dylox).  You can effectively treat JB with a curative insecticide until about mid-August.  By spring, the grubs are too large to effectively treat them.
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