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.
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