First of all. . .
Spider mites are definitely not insects. They definitely are arachnids. All adult and nymphal spider mites have a body consisting of a single oval-shaped abdomen, eight legs, and a head. Spider mites lack wings and antennae. Larval mites have only 6 legs, but are otherwise similar. Spider mites have needle-like mouthparts that they use to suck fluid from individual plant cells. Males are smaller than females and have a slightly pointed abdomen. Spider mites are generally warm-weather pests, and do best in temperatures 75 degrees Fahrenheit and above, especially if it is dry out.

Reproduction
Spider mites reproduce quickly, and require a period of only 5 - 20 days for full development, depending on conditions. Because of the short development time, multiple generations can occur over one season. Spider mites go through an egg stage, a larval stage, and two nymphal stages before reaching adulthood.

Types and Identification
Two-spotted
The two-spotted spider mite is considered a warm-season mite. It has an extremely wide host range, and has been reported to occur on more than 300 species of host plants. They range in color from light yellow to dark green or brown, with two clearly visible spots on either side of the abdomen. This species does best when conditions are very hot (up to 100 degrees) and very dry. They become active in April or May, and if conditions remain good, may continue to feed and reproduce through the beginning of fall. This variety spends the winter as females living in the soil or in any other closed, dry space they can get to.

Spruce
The spruce spider mite is considered a cool-season mite. These mites attack a wide variety of coniferous trees, not just spruce as the name would imply. Color varies from green to brownish red. This variety becomes active as early as March depending conditions, though as temperatures reach 90 degrees they tend to back off. Once summer temperatures back off again in fall, spruce spider mite populations may rebound, and they will continue to feed throughout a mild winter. Once the temperature gets too cold, this species overwinters in the egg stage.

European Red
European red spider mites are a warm-season species. They occur most commonly on flowering trees, such as apple and and cherry. All stages of this species are a brick red color, except for the overwinter egg stage, which has a brighter red color.

Southern Red
Another cool-season species, similar to the spruce spider mite. It concentrates mainly on broad-leaf evergreens such as holly and rhododendron.

Banks Grass
Similar to the two-spotted spider mite, and another warm-season species. Commonly a corn pest. This species overwinters on various grass species, and can also be a problem for grassy crops such as wheat.

Distribution
Varieties of spider mite occur throughout the world. Infestations can occur wherever conditions become warm and dry. While generally a warm weather pest, some of the cool-season species can reproduce at temperatures as low as 55 degrees.

Damage
Spider mites feed on the undersides of the leaves of the plants they infest. When the spider mites feed, the leaves will become speckled green or white from the damage. Eventually the leaves will yellow or brown and then die. While infestations are not neccessarily fatal for the plant, severe infestations can be, and may stunt smaller plants. Late season damage to conifers may not become noticeable until the heat from the next summer season dries the needles and makes it obvious.

Treatment
Many treatments are available for spider mite infestation. Some of the most common:

Water -- cheap, easy, and safe!
Heavy rains tend to knock mites off of leaves. If your infestation is minor, you may be able to control it simply by spraying down all affectected plants with water.

Predatory control:
A variety of predator mites are available. Controlling with this method is difficult, however. Steps must be taken to ensure that the wrong mite is not used, as some of these mites will also damage crops. P. persimilis, for example, will snip off the stems and leaves of carnations. Conditions must also be considered. P. macropilis will do much better than P. persimilis in high humidity, but P. longpipes will do better in high temperatures at low humidity. Finally, this method must be administered at the right time. If the predator mites are released too late, they will not be able to control the spider mite population alone.

Chemical Control
There are a wide variety of insecticides available for controlling spider mites. Unfortunately many of the over the counter types available are both excessively toxic and fairly ineffective. Bad combination. The most effective treatments are miticidal oils and soaps. These require contact with the mites to be effective, so the plant has to receive good coverage. Oils and soaps have very little residual effect, which is good for low toxicity, but does mean that multiple doses will be required. These are actually effective against both adults and eggs. Other pesticidal options include pesticidal sprays. Acephate (Orthene), dimethoate (Cygon), chlorpyrifos (Dursban), disulfoton (Di-syston), and malathion are all available over the counter, but are considered weak miticides. Safer's is another option, but it may be of limited availability. Two-spotted mites are highly resistant to many insecticides. If these mites are your problem, you can rotate between Pentac, Mavrik, and Avid. Please follow the directions for these chemicals, and make sure you rotate. Resistance is a problem for everyone.

Prevention
As with any problem, the best solution is usually prevention. Inspect and quarantine any new plants you buy. If you notice mites before you purchase the plant, don't buy it now. Just wait and go somewhere else. It might seem like a hassle now, but it's a lot easier and cheaper than fighting back a new infestation. Quarantine ensures that you didn't buy a plant with eggs on it. Keep the plant by itself for a week or so and make sure nothing hatches. Once you're sure your plant is mite-free, go ahead and add it the rest of your stock.

Notes:

N0b0dY says re: spider mite - seaweed spray is a non-toxic method for fighting mite infestations. napalm is also effective, if somewhat extravagant.

futilelord says re spider mite: safer is still available, least in NM, also you should suggest neem oil, or something that contains neem oil, derived from the neem tree so it's relativly natural, plus that stuff can also take care of fungus as well as some insects

This node was written in response to a wertperch bounty found at Everything's Most Wanted.
    Sources:
  • http://ipm.ncsu.edu/current_ipm/98PestNews/98News6/ornament.html
  • http://ohioline.osu.edu/hyg-fact/2000/2012.html
  • http://www.uky.edu/Ag/Entomology/entfacts/trees/ef438.htm
  • http://www.uwex.edu/ces/wihort/pests/TwospottedMite.htm
  • http://www.evergreengardenworks.com/mites.htm
  • http://www.ext.colostate.edu/pubs/insect/05555.html
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