I'm writing this as a guide for controlling spider mites because I recently had a really MAJOR problem with them and I found a LOT of bad info floating around.
Spider Mites
Spider Mite damage results in a "stippling" or spots that appear on the leaves. Often it is not as pronounced as in the picture and the areas where mites are most concentrated just turn brown, dry up, and crumble.
Spider mites breed very quickly and once their population gets high enough the rate of damage increases exponentially. With no control spider mites can quickly defoliate and kill a large cannabis plant in less than two weeks.
Mites themselves are barely visable to the naked eye however with a jewler's loupe they can be seen with translucent bodies having two black spots. They are usually only on the undersides of the leaves unless you have a bad infestation.
Prevention
If you area reading this then probably, like me you already have mites. I still think it is important that I discuss prevention so that when you do get them under control they don't come back!
For indoor gardens pests will likely enter the grow room in larger numbers during late summer and fall months to escape weather extremes. You can cover your intakes with silk-screen and spray it down with a miticide such as Avid or Floramite. Keep in mind that the screen with reduce intake surface area by up to 70% resulting in reduced air-flow unless you expand your intake surface area by about 70% as well (AKA bigger intake).
Mites also hitch a ride on your buddies clones into you grow room. I recommend treating any clones you get from the dispensary, good buddy, top grower, ANYBODY. Use Avid or Floramite as a one time preventative kill-all.
If you go your local hydro store change your clothes before you enter your garden. Everybody and their mother comes in there with spider mites frantic and spreads them around for you to take home. I also hang a few clean lab-coats outside my room now days just to be extra safe.
Mite Control
Finally what you have been waiting for, how to kill these bastards! Mites are VERY hardy, I dare say they are more hardy than cockroaches. If there was a nuclear winter I'm sure mites would survive quite happily.
There are a million and one products on the market today that "claim" to work. Unfortunately few provide adequate control for anyone other than a very small hobby grower who found three mites on their plant. I'll try to go over a few of the most popular ones. If you have a product in mind I have not listed feel free to PM me or post your question.
Neem Oil: Seems to suppress small populations on healthy plants but mites will slowly work their way up to the east side and eat your plants. If you are using IPM(Integrated Pest Management) and other controls as a preventative it should be applied weekly for best results.
Neem Oil Concentrates: Not all are created equal some are better than others Azatrol works well to suppress mite populations and can be used as a safe control for flowering plants not already heavily infested. Does not need to be applied as often.
Pyrethroids: Probably the most common product used as a broad range pesticide. Kills everything but mites seem to only be slowed down a bit and subsequent generations quickly build a high tolerance to this pesticide. There are two types artificial and organic pyrethroids. Organic pyrethroids quickly break down and become inert providing little control of anything but very small populations. If you use these pyrethroids I would recommend an artificial because they have longer residual effects. These are contact only insecticides and DO NOT kill eggs requiring multiple applications. Overall these are not very effective at controlling mites and I would only recommend them if you are late in flower or have a minor problem.
Insect Soaps: Don't even bother, SAFER soap works great outside where natural predators can work with you but inside it hardly phases the mites. Don't let the damn hippies tell you that all natural will work with spider mites too, trust me I wish it did. *Some soaps contain Pyrethroids and the combined action of the soap seems to provide mild control.
Nicotine Tea: If you are on a really tight budget collect some old cigarette butts and boil them in water for about 20 min. Let it cool strain out the butts mix it with a bit of soap and spray all the leaf surfaces really well. Wear gloves nicotine can make you feel pretty sick if you get enough and can be absorbed through your skin. It seems to do a pretty good job at killing mites but breaks down with-in a day requiring repeat applications. For heavy infestations they build up a tolerance after about a week. May work well if you catch the mites early and spray often.
Spider Mite Predators: The epitome of natural control, if used with a strong prevention system in a clean grow room I hear they are effective. It often takes up to a year for their populations to become mobile and effective requiring multiple releases. They also need food! They can survive on pollen for a little while but eventually if their are no spider mites to eat they die off. They are also VERY expensive upwards of $200 for a killing combo of predator mites.
Avid: This is one of the only TRUELY effective products for spider mites. This is an industrial pesticide and should be used responsibly. It is pretty pricey about $300 a quart but you can find smaller amounts being sold on e-bay for $15-30 USD. You don't need much 1-2.5 ml/gal will do it. If used properly it will kill all the mites and eggs. It is also stored in leaf tissue for 28 days so I would not use it on late flowering plants. For heavy infestations spray again a week later. No contact is required due to the chemical's systemic movement. ROTATE with another miticide or you will end up with a super mite that is impossible to kill.
Floramite: Another VERY effective product, uses a different activity than Avid to kill mites but is also stored in leaf tissue for up to 28 days. Kills both mites and eggs. I use this product in a one month rotation with Avid. About the same price as Avid. Remember if you use this ROTATE! DOn't make a super-mite for everyone!
That covers the big ones hope I helped you kill these frustrating insects!