Question of the Week
Answer: Spider mites on tomatoes can be either web-spinning spider mites, which can be seen with the naked eye, or tomato russet mites, which are invisible to the naked eye but leave plants with a "bronzed" appearance. It would be best to confirm the particular type of mite with a local Extension agent in order to manage it most effectively, since the different types of mites will prefer different hosts and may react differently to control measures.
There are several strategies that can be integrated to limit spider mite populations, including cultural controls, biological controls, and various types of sprays.
Dusty conditions can help cause spider mite outbreaks. You can reduce the dust that collects on plants by limiting the speed of vehicles, limiting vehicle access, or watering the roads around the planted area on a regular basis. Providing plants with adequate water also reduces stress on the plants and allows them to better withstand spider mite infestations. Spraying the plants periodically with high pressure water sprays will help to remove both dust and mites.
If chemical sprays are not used, the spider mite's many natural enemies will generally provide good control. Perhaps the most important natural enemies are predatory mites of various types. These mites are about the same size as spider mites, but they move about more quickly on their longer legs. Predatory mites are commercially available and information about insectaries that sell them in your area can be obtained from: California Department of Pesticide Regulation, P.O. Box 942871, Sacramento, CA 94271-0001; (916) 324-4100. Ask for a free copy of "Suppliers of Beneficial Organisms in North America," or download it from the Web at http://www.cdpr.ca.gov/docs/ipminov/bensuppl.htm.
Other natural enemies of spider mites include general predators such as minute pirate bugs, bigeyed bugs, and lacewing larvae. Mite-specific predators include the sixspotted thrips, Scolothrips sexmaculatus; the larvae and adults of the spider mite destroyer lady beetle, Stethorus picipes; and larvae of certain flies including the cecidomyid Feltiella acarivora. In order to maintain significant populations of these beneficial insects on your farm, there must be sufficient habitat for them--pollen and nectar sources as well as prey on which they can feed. Options for providing such habitat are described in more detail in the ATTRA publication, Farmscaping to Enhance Biological Control, available at https://attra.ncat.org/attra-pub/summaries/summary.php?pub=145.
There is a range of commercially available materials that are formulated to be applied to foliage and control mite populations. Care should be taken not to use soaps or oils on water-stressed plants or when temperatures exceed 90°F (1). Both of these materials may be phytotoxic to some plants and should be tested out on a portion of the foliage several days before applying a full treatment. Most of the products listed below must contact mites to kill them, so excellent coverage, especially on the undersides of leaves, is essential, and repeated applications may be required. Sulfur dust or spray can be used on some vegetables but will burn cucurbits. Do not use sulfur dust if temperatures exceed 90°F, and do not apply sulfur within 30 days of an oil spray (1).
Sulfur dusts are skin irritants and eye and respiratory hazards. Always wear appropriate protective clothing.
1. UC Pest Management Guidelines. 2000 (revised 2011). Spider Mites. http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7405.html
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