Spider mites are members of the Acari (mite) family Tetranychidae, which includes about 1,600 species. They generally live on the under sides of leaves of plants, where they may spin protective silk webs, and they can cause damage by puncturing the plant cells to feed. Spider mites are known to feed on several hundred species of plant.
Spider mites are less than 1 mm in size and vary in color. They lay small, spherical, initially transparent eggs and many species spin silk webbing to help protect the colony from predators; they get the 'spider' part of their common name from this webbing. Hot, dry conditions are often associated with population build-up of spider mites. Under optimal conditions (approximately 80ºF (25ºC)), the two-spotted spider mite can hatch in as little as 3 days, and become sexually mature in as little as 5 days. One female can lay up to 20 eggs per day and can live for 2 to 4 weeks, laying hundreds of eggs. A single mature female can spawn a population of a million mites in a month or less. This accelerated reproductive rate allows spider mite populations to adapt quickly to resist pesticides, so chemical control methods can become somewhat ineffectual when the same pesticide is used over a prolonged period.
The best known member of the group is Tetranychus urticae (the glasshouse red spider mite, or two-spotted spider mite), which is common in tropical and warm temperate zones, and in glasshouses. Other species which can be important pests of commercial plants include Panonychus ulmi (fruit tree red spider mite) and Panonychus citri (citrus red mite).
Spider mites, like hymenopterans and some homopterous insects, are arrhenotochous: females are diploid and males are haploid. When mated, females avoid the fecundation of some eggs to produce males. Fertilized eggs produce diploid females. Unmated, unfertilized females still lay eggs, that originate exclusively haploid males.
Chemical control of spider mites generally involves pesticides that are specifically developed for spider mite control (miticides or acaricides). Few insecticides are effective for spider mites and many even aggravate problems. Furthermore, strains of spider mites resistant to pesticides frequently develop, making control difficult. Because most miticides do not affect eggs, a repeat application at an approximately 10 to 14 days interval is usually needed for control. Since an egg can develop into a mature spider mite able to lay eggs of its own in as little as 9 days, more frequent application may be required in hot, dry conditions.