European red mites in 2009

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

By all standards, 2009 has not had the weather conditions that we usually think of that would drive European red mite populations to the levels we are seeing in some blocks around the Grand Rapids area. It’s been mostly cool with some very heavy rainfall events – not hot and dry, which typically favors mites. There are several thoughts as to what might be happening and no real answers at this time to help growers get a handle on some of these blocks with extremely high populations that just aren’t being knocked back very well by summer miticides.

If we go back to the summer of 2008, it was hot and dry, and the apple crop was very low. Growers let European red mites get a little higher than usual because the crop was low, and it was thought that the thresholds could be pushed a little higher with no detriment to the trees ability to produce carbohydrates. This should have saved growers money and allowed beneficials to build up to help in 2009.

In spring 2009, some blocks started out with surprisingly very high numbers of European red mites at petal fall. It was difficult to use petal fall miticides that require the use of oil because high apple scab pressure called for the use of fungicides which can’t be used around oil. This early presence of European red mites in apple slid the summer forward, and growers were forced to put on typical summer miticides much earlier than normal. Most problem mite blocks have had two or, even three, summer miticides applications already. Yet, the European red mite numbers don’t seem to be declining at all. It’s logical to ask if resistance to miticides is the issue, but that would require testing to answer.

Again, it’s not been a “hot” year weather-wise for European red mites, yet they keep marching on. Of the most interest is the apparent lack of beneficials in these high re mite populations. Where are they? What’s limiting their appearance? Something has removed the predators of European red mites from the equation and that seems to be allowing them to build.

While this article won’t provide a solution to controlling European red mites for 2009, I hope it will call for reflection on what caused them to be such a problem in 2009. Growers are encouraged to look back at the spray materials they used in the summer of 2008 for something that might have negatively affected European red mite predators. There could be several answers to the problem with European red mites in 2009, and hopefully the beneficials will make a comeback for 2010.

Dr. Gut's work is funded in part by MSU's AgBioResearch.

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