General Information on Mites

In the San Joaquin Valley, one of the most effective predators to release in orchards and vineyards is the Western predatory mite, Galendromus occidentalis (Nesbitt).  It is native to western North America and found naturally occurring in deciduous orchards and vineyards. Another highly effective predatory mite for the SJV and many other field and greenhouse settings throughout the country is the Cali predatory mite, Neoseiulus californicus. Most native G. occidentalis and N. californicus are killed by commonly used pesticides. The Sterling Insectary strain of these predatory mites have been breed to be resistant to many commonly used fungicides, insecticides, and miticides. This allows implementation of the Sterling strain of predatory mites into many conventional crop management programs without the fear of losing your predatory mite investment when it comes time to treat for other pests. G. occidentalis is most effective against web-spinning mites including the Pacific, Two-spotted and Willamette spider mites and can be an effective predator of European red and Citrus red spider mites. N.californicus is effective against the web-spinning mites and persea mites.

These predators do not feed on foliage or become a pest; it is an obligatory predator, and if spider mites are not available, it will starve or migrate out of the field. This means that miticides must be properly timed and appropriate rates applied to maintain low populations of predators and spider mites on a continuing basis.  One-quarter to one-half label rates of OmiteÓ or AgrimekÓ are usually effective in controlling out of balance predator to prey ratios.

Commercial mass production of Galendromus occidentalis and Neoseiulus californicus are typically produced by growing the predators on bean plants. If a bean plant or part of a bean plant is placed into the crotch of a tree or on the canopy of a vine, the predators will walk off the wilting foliage and into the tree. Once they find spider mites they settle down and reproduce. They can move from tree to tree by walking and by wind currents. These predators can also be purchased in bottles for ease of shipping and handling. The bottle is filled with a carrier media (vermiculite) and can be sprinkled or flown on to the crop.

The most practical sampling method used is the presence-absence sequential sampling method, developed by the UC system. This sampling method will provide an estimate of when to release predatory mites and also if a follow up treatment is required. The sampling should take place once a week from late March through September. It is often useful to graph your samples.

Problem areas within a field, such as areas of sandy soil or areas near dusty roads, should be treated and sampled separately. These areas usually have a greater potential for spider mite outbreaks. The rest of the orchard or vineyard should be sampled together.

Release Rates & Timings

There are three criteria that must be addressed when releasing predatory mites in your field. One is to release them early in the season so that you can take advantage of their ability to increase rapidly. The second is that they must be released when there are some spider mites in your crop so that they do not starve and disperse out of your crop. The third criteria is that you understand the chemicals that the predatory mites are resistant to and manage pesticide application accordingly. Spider mites often migrate into crops during late March through May.

When 5% to 15% of your sampled leaves are infected with spider mites, it is time to release predatory mites. A release rate of between 2,000 – 5,000 predatory mites per acre is usually enough to initially inoculate your field. Careful follow up sampling must be performed to determine if another release is necessary. If your presence absence sampling shows that your initial release is not controlling your spider mites, then a second release of between 2,000 – 5,000 can be used to control the spider mite.

When a Miticide Treatment is Needed

By following the presence-absence sampling method, it is easy to determine when a ¼ to ½ labeled rate of a miticide is needed.  In the presence of predators, when 45% -50% of the leaves are infected with spider mites, it is time to treat with a lower than labeled rate of a miticide. When the percent of leaves infested with spider mites approaches the percent infested with predators, the spider mite population will decline soon. If the proportion of leaves infested with predators is ¼ to 1/3 that of the proportion of leaves infested with spider mites, the spider mite population should decline in two to three weeks. When spider mites reach the treatment threshold, a miticide spray is needed. A lower than label rate of miticide should always be used to preserve some food for the predators. The lower than label rate also helps to manage the spider mites resistance to miticides. If you have any questions, please feel free to call. Click here for our contact information.