MSU Extension Nursery

Editor's note: This article has been updated on March 8, 2013.

It is only March 6 and already this year we have received several reports of growers struggling to control spider mites. In some cases, the grower has tried one of our standard miticides like Avid, Floramite or Hexygon, and the spider mites are still there.

Avid, Floramite and Hexygon were considered to be some of the best miticides available to greenhouse growers for many years. One application would nearly eliminate spider mite problems for six weeks. Some growers are saying this is no longer the case. Although it is possible that some populations of spider mites have built resistance to one or more of these products, we don't know this for sure until different populations of spider mites are tested for susceptibility to each product in a research test. Even if spider mites in some greenhouses are resistant, it is likely that resistance levels vary considerably among greenhouse locations, and that these products may be working well for some growers. It is difficult to blame anyone for a resistance problem. If history is any predictor of the future, it is likely that spider mites will develop some level of resistance to miticides that are heavily used for many years.

So, if you are having problems getting control of spider mites in your greenhouse, what do you do?

The best option for getting control of spider mites that do not appear to be susceptible to a miticide that you are using is to choose a relatively new miticide with a different mode of action. You can look at the Michigan State University Extension greenhouse wallchart (E-2696) or the new smartphone app for this, but for an example, let's say you did not adequate control with one of our top miticides: Avid, Hexygon or Floramite. First, check on the classification of the miticide by its mode of action listed in the wall chart. So, for these three products, the classes are:

• Avid (class 6)
• Hexygon (class 10A)
• Floramite (class 25)

Now look for miticides in a different chemical class. Some of the miticides with a mode of action that is different from Avid, Hexygon or Floramite are:

• Akari (class 21A)
• Sanmite (class 21A)
• Judo (class 23)
• Kontos (class 23)
• ProMite (class 12B)
• Pylon (class 13)
• Shuttle O (class 20B)

Fortunately, we have quite a few new miticides to choose. Use a standard high volume sprayer (not a fogger, and not a total release product) to distribute the miticide spray throughout the plant canopy. Spider mites are mostly on the undersides of leaves, and may escape fogs that tend to deposit mostly on the upper surfaces of leaves. If there is no resistance to the miticide, one application with good coverage should suppress spider mites for four weeks. If you are buying plugs, cuttings or starter plants, it may also help to ask the supplier what they have been using for spider mite control. They may already be aware of some resistance problems.

If Michigan growers continue to have spider mite problems, we may want to consider working together on a spider mite test next year to compare how well all of these products work. Also, in the future, some growers may want to consider biological control using predator mites. Quite a few Michigan growers are already using biological control. This is a good choice for a new crop that has never been sprayed with a pesticide, but biological control is not a good idea for crops that have already been sprayed. The predator mites tend be very sensitive to miticides and insecticides. This can also be a problem when releasing predator mites on cuttings or plugs that have been treated by the supplier. One product that is more toxic to spider mites than predator mites, and therefore may be helpful in transitioning from chemical control to predator mite control, is Hexygon.

A final word on resistance: I have used Avid, Hexygon and Floramite as an example in this article because they have been widely used in floriculture greenhouses for spider mite control. However, these products may be working very well in many, if not a majority, of greenhouses. The only way to know for sure is to test spider mites collected from different greenhouses and compare their susceptibility to selected products.

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