The Importance of Integrated Pest Management for the Successful Use of Predator Mites to Control Spider Mites

Mark Morris

Spider mites are among the most destructive agricultural pests throughout the world. Because of recent restrictions on the use of conventional synthetic insecticides, and negative public perceptions of their use, biological control of spider mites is becoming an increasingly attractive option for many crops. Moreover, biological control is often less costly than conventional miticides, an important benefit in light of global competition in agriculture. Of the many natural enemies that feed on spider mites, predator mites are among the most effective. We believe that to manage predator mites most effectively, growers need to implement Integrated Pest Management (IPM). There are many definitions of IPM. The U.S. Peppermint and Spearmint Industry has adopted the following definition: IPM is the intelligent selection of all pest control options in a manner than is both cost effective and least harmful to non-target organisms and the outside environment.

Six IPM tactics that improve the chances for successful implementation of biological control are:

First, education and training is a must. This approach helps to avoid unrealistic expectations by program participants, a major reason why biological control is dropped when original expectations are not met.

Second, aspects of biological control as an IPM tactic must be understood. Examples include the assessment of crop suitability, identification of promising natural enemies, quality control during their mass production, and cost effective ways to apply them to the crop. With this knowledge, the best strategy for implementing biological control can often be determined.

Third, treatment thresholds (critical densities) for spider mites on specific crops must be known. Without this information the level of spider mites that can be tolerated before intervention with a miticide becomes necessary is unknown. To provide food for predators, a crop usually must tolerate at least low levels of spider mites. Determining the level of predators necessary to achieve control of spider mites before economic damage occurs is also important. Once critical densities of spider mites and predators are known, crop monitoring is needed to accurately determine spider mite and predator mite levels. Crop monitoring is also necessary to determine if predators are needed and, if they are required, how many are needed and when they should be applied. Because biological control is often a slow process, field monitoring is also necessary to evaluate progress and determine if further intervention may be necessary.

Fourth, crop cultural practices must be evaluated for their effect on biological control. For example, selecting a plant variety that is not too susceptible to spider mites may allow predators the opportunity to control these pests without further intervention with miticides.

Fifth, because natural enemies may provide only partial control of spider mites, miticides will continue to be an indispensable part of mint IPM. Because miticides may be harmful to predators, however, they should only be used when absolutely necessary. Selective miticides should be used first if they are effective and economical. Selective pesticides are those that are more toxic to the target pest than to its natural enemies. Even if selective alternatives are not available, selectivity may sometimes be achieved by altering the timing and method of application, or by choosing a different pesticide formulation.

Finally, pest managers must be aware of how other crops or crop pests in the agroecosystem of interest can affect biological control of spider mites. For example, propane flaming mint fields in late summer to suppress the soil pathogen, Verticillium dahliae, is harmful to predator mites and can result in damaging levels of spider mites during fall or the following spring. Adjacent crops can also affect biological control of spider mites. Populations of spider mites can rapidly increase on sweet corn, then disperse downwind to mint, potato or other susceptible crops.

Mark Morris
A.M. Todd Company
PO Box 149
Jefferson, OR 97352

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