Fruit Crops News

Biological Control of Mites in Midwest Apple Orchards
Part 4: Approaches to Implementation


Editor's note: this is the fourth and concluding article on biological control of pest mites in apple orchards.

In Parts 2 and 3 of this series I outlined the types of natural enemies important in orchard mite biological control and the impacts of pesticides on these beneficials. In this article I will attempt to lay out an approach to the development of an integrated mite management program based on natural enemies but relying, where necessary, on the use of miticides.

Essential to any Integrated Pest Management program is the adoption of regular pest and natural enemy monitoring practices. A first step is simply being able to recognize the various species. For help with this, refer to Common Tree Fruit Pests, North Cent. Reg. Extension Pub. 63, published by Michigan State University Extension Service. Monitoring involves the routine sampling of foliage, once or twice per week, to determine the numbers of mites and predators present. Although suggested sampling protocols vary from state to state, the following is representative. Sample 100 leaves per block of trees no larger than 10 acres. A standard sample is 10 arbitrarily-selected interior and exterior leaves from 10 trees per block. In mixed blocks, be sure to sample the more susceptible varieties. The mites can be counted in the field with a hand lens, or returned to the office or lab and counted with a microscope. This count will give you an average number of mites and predators per leaf. Sample the same blocks each time and keep records of your counts! While you are sampling, also keep track of incidental predators such as lady beetles, pirate bugs, and lacewings that can also feed on spider mites.

Mite management decisions can be based on the pest:predator ratio. I will only discuss the predatory phytoseiid mites; guidelines are also available for Stethorus, the little predatory lady beetle. Again, the following are generalities; the specifics can vary depending upon climatic conditions, susceptibility of specific varieties, and amount of apparent injury. A ratio that is usually successful is one predatory mite to 5 spider mites; however, this ratio is rarely seen. A ratio of 1:10 is more common. At low to moderate spider mite levels, this ratio will often result in control. Ratios below 1:15 will result in successful biological control less that 50% of the time; however, if spider mites are still below threshold levels, another sample should be taken in 4-5 days to reassess the situation. If the pest mite numbers are declining and the predators increasing, successful biological control may still be achieved. Again, compare the results from successive monitoring periods to see changes in numbers of both the pests and the predators.

Pesticide management is the other important component of a mite IPM program. If it is necessary to use a miticide, use one that is compatible with predators (see Part 3). Also, reduce the use of broad spectrum insecticides by carefully monitoring all orchard pests. Generally, organophosphate insecticides are easier on beneficials than carbamates and synthetic pyrethroids; use materials from these latter groups only if absolutely necessary, recognizing that their use may result in mite upsets. If possible, spot spray to control pest problems; mite predators will be conserved in untreated areas. Consider using other pest management practices that do not use disruptive pesticides, such as the microbial insecticide Bacillus thuringiensis for caterpillar control, mating disruption for codling moth, or trapping for apple maggot. Also remember that predatory mites overwinter on the orchard floor; if needed, pesticides can be more safely used early in the season (before mid June), especially if overspray or runoff onto the orchard floor does not occur. Finally, remember that other pesticides can also impact predators. For example, the fungicide benomyl may restrict the reproduction and effectiveness of predatory phytoseiids if used in mid season.

This series of articles is meant to be an introduction to orchard mite biological control. The actual practice takes a commitment on the part of the grower or pest manager. Other resources are available to help you get started. Contact your local extension service for further help. Some areas have commercial pest management consultants that will conduct the monitoring and provide recommendations. The first two publications cited at the end of Part 3 of this series will also be helpful. In conclusion, Integrated Mite Management based on biological control and wise use of pesticides can be a viable practice in Midwest apple orchards.

- Dan Mahr, University of Wisconsin - Madison


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