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.
[edit] Countermeasures
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-day 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.
Various insects and predatory mites feed on spider mites and provide a high level of natural control. One group of small, dark-colored lady beetles known as the "spider mite destroyers" (
Stethorus species) are specialized predators of spider mites. Minute pirate bugs (family
Anthocoridae), big-eyed bugs (
Geocoris species) and predatory
thrips can be important natural enemies.
A great many mites in the family
Phytoseiidae are predators of spider mites. In addition to those that occur naturally, some of these are produced in commercial insectaries for release as biological controls. Among those most commonly sold via mail order are
Galendromus occidentalis,
Phytoseiulus persimilis,
Mesoseiulus longipes,
Amblyseius fallicus, and
Neoseiulus californicus. Predatory mites eat adult mites, their eggs, and all developmental stages between. Predatory mites can consume as many as 5 adult spider mites per day, or 20 eggs per day (as many eggs as a female spider mite can lay).
Neem oil may provide control, when combined with a suitable surfactant and diluted with water. As with chemical control, repeated applications are required.
