Assessing Spider Mite Natural Control in Apple Orchards

There are two important components of natural control of spider mites in apple orchards: predation and rainstorms. These natural controls can often substantially or totally control pest mites such as European red mite and twospotted spider mite. Both of these natural controls are easily assessed using routine sampling methods. Remember that the threshold for mites is generally considered 10-15 per leaf, especially if leaf discoloration is evident.

Assessing predation. Predators of orchard mites were discussed in a series of earlier articles in MBCN (Vol.1, Nos. 1, 2, 3, 4). Throughout most of the Midwest, phytoseiid mites will be the most commonly encountered predators. Sample pest and beneficial mites weekly. Pick 10 leaves per tree on 10 trees per block (10 acres or less). If you are unfamiliar with the distinction between spider mites and phytoseiid mites, it will be helpful to do your counts with the aid of a low power microscope, although a 10-15x magnifying glass will suffice. Predator mites are about the same size as spider mites, but tend to be shinier, more pear shaped, and faster moving when disturbed. Once you become familiar with the distinction, it will be possible to do your counts directly in the field. Keep track of your counts by tree (average number mites per leaf) and keep records from week to week. Also, keep track of pesticide applications that may interfere with the predators, such as the fungicide benomyl or insecticides in the carbamate or pyrethroid classes. Remember that some apple varieties (e.g. Delicious) are more susceptible to mites than others; therefore, all of your trees should be of the same variety. Make a graph of mite counts vs. time. How many predator mites did you find at the beginning of the season? At the end of the season? As predator mite numbers increased what happened to spider mite numbers?

Assessing the impact of rainstorms. Heavy, driving rainstorms will wash many mites from leaves, especially on small trees or well-pruned trees with an open canopy. Keep track of significant rains to determine their impact on mite populations. Did hard rains reduce mite numbers at all?

You may wish to plan a field workshop during the peak of mite activity in early to midsummer. You can present your data that may show the impacts of natural controls and pesticides on the mite populations. Further, you can use this opportunity to train growers how to recognize beneficials and take their own counts for both pest mites and predators.

- Dan Mahr, University of Wisconsin - Madison

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