Biological Control
Spider mites have many natural enemies, which limit their numbers in many landscapes and gardens, especially when undisturbed by pesticide sprays. Some of the most important are the predatory mites, including the
western predatory mite,
Galendromus (=
Metaseiulus) occidentalis, and
Phytoseiulus species. Predatory mites are about the same size as plant-feeding mites but have longer legs and are more active. Various insects are also important predators—the
sixspotted thrips (Scolothrips sexmaculatus), the larvae and adults of the
spider mite destroyer lady beetle (Stethorus picipes), the larvae of certain flies including the
cecidomyid Feltiella acarivora, and various general predators such as
minute pirate bugs,
bigeyed bugs, and
lacewing . The purchase and release of predatory mites can be useful in establishing populations in large plantings or orchards, but the best results are obtained by creating favorable conditions for naturally occurring predators—for instance, by avoiding dusty conditions and pesticide sprays.
The major predator mites commercially available for release are the western predatory mite and
Phytoseiulus. The western predatory mite is more effective under hot, dry conditions. These predators do not feed on foliage or become pests; thus if pest mites are not available when predatory mites are released, the predators starve or migrate elsewhere. If you wish to establish predators in a heavily infested orchard or garden that has few predators, use a soap spray or selective miticide to bring pest mites to a lower level and then release predatory mites. A good guideline is that one predator is needed for every ten spider mites to provide control. More than one application of predatory mites may be required if you want to reduce pest populations rapidly. Concentrate releases in hot spots where spider mite numbers are highest. Once established on perennials, predatory mites may reproduce and provide biological control indefinitely without further augmentation unless nonselective insecticides are applied that kill the predators. For information on where to buy these predators, write or call California Department of Pesticide Regulation, P.O. Box 942871, Sacramento, CA 94271-0001, (916) 324-4100, and ask for a free copy of their leaflet "Suppliers of Beneficial Organisms in North America," or view it on the World Wide Web at
http://www.cdpr.ca.gov/docs/ipminov/bensuppl.htm.