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Insects, mites, plant pathogens and weeds all can become resistant to chemicals used to control them over a period of time. Some say that greenhouses are among the primary sources of resistant pests because of the intense pesticide application schedule often carried out in such enclosed environments. Although some studies have demonstrated that spider mites (as well as some insect species) in greenhouses are generally more resistant to pesticides than mites on outdoor crops, there is little evidence that any greenhouse pesticide resistance moves outside of the greenhouse.

Spider mites are among the pests well known for their ability to develop resistance to pesticides used to control them. This is partly because of their biology, their wide host plant range, and the difficulty of contacting them with pesticides. Because we cannot change any of the characteristics about the mites, we need to think about pesticide resistance management.

The key to managing pesticide resistance is using pesticides wisely. Wise use includes avoiding unnecessary applications and overuse of the same kind of chemistry. One of the best ways to do this is with a pest scouting and monitoring program. Knowing what pests are there, where they are and some estimate of the infestation level will help a control program at the beginning and throughout the entire crop production cycle. When possible, use alternatives to pesticides to reduce pest numbers. In the case of spider mites, alternatives may include eliminating weeds that harbor mite infestations, maintaining greenhouse temperature and humidity within moderate ranges, and introducing predatory mites.

Advising growers to minimize pesticide use may seem unusual coming from a chemical company, but it’s in our interest as well as yours to avoid unnecessary pesticide applications. Our job, as chemical suppliers and growers, is to apply pesticides in such a responsible way that resistance development is delayed as long as possible in order to preserve the effectiveness of pesticides. We would like to have our products (and those from other companies as well) around, and have them remain effective for a long time.

Pesticides will be needed for spider mite control in most greenhouses – especially during hot weather. In addition to minimizing pesticide use, another principle of pesticide resistance management is to not depend on one or two kinds of chemistry for control, but to rotate among at least three (preferably four) different chemical modes of action. Fortunately, at this time, we have these different choices of modes of action available to us in most areas of the United States. It’s a good thing that so many different products are available because label directions on many of them, especially those registered recently, have specific instructions on the number of applications permitted. This is, of course, to minimize exposure to one mode of action.

Finally, a chemical application is only as good as the coverage obtained. Most of the products need to contact the mites or the leaf surfaces on which they walk and feed, to be effective. Judo, Pylon Avid and TetraSan have translaminar activity; i.e.the active ingredient moves from upper to lower leaf surfaces – and vice-versa - after application. This helps a bit if coverage is less than perfect, but good coverage will result in even better control.

Here are two pesticide rotation programs that can be used to control spider mites while minimizing exposure to a single mode of action. The first one is for a low mite infestation and the second is for controlling high mite numbers. The application frequency will need to be adjusted, depending on temperatures and other environmental conditions. Read the product label before using for application directions, plant safety information and application frequency. Do not apply any product more often than allowed on the label!

I. Low Initial Infestation

II. High Initial Infestation