The Spider mite Life cycle is in 5 stages Fom Eggs to Larva to 1St nymph stage to 2nd Nymph stage to Adult. This life cycle depends on temps. The entire cycle can be over 10 days in periods of higher temps. As temps and daylength decrease females go into hibernation and crawl into a dark place until better conditions prevail.
Why is it important to know about their life cycle you ask?
With this knowledge it becomes much easier to eradicate them.
Two factors in orchid culture facilitate an explosion of spider mites. One is lack of Humidity. They flourish in low humidity growing areas. The second is plants that are stressed due to chronic lack of water. So that is something that is just as important as which pesticide to use.
Which pesticide to use depends on situation and which stage of the life cycle. If you are growing in a home Using a pesticide meant for a greenhouse is not a good idea. There are lots of pesticides out there but the market for the home gardener is relatively small. The more toxic stuff is available only to growers who have a pesticide applicators licence (in Canada) Those growers also have access to more info on toxcicity, which stage of the life cycle it is effective on and whether it is systemic or non systemic killing on contact and if it acts over several days or if as soon as it is dry it stops being effective.
Personally I favor products such as Trounce or End All in my greenhouse It is considered an organic product and works against a broad range of insects. It kills on contact but isn't effective once it dries. Plus its toxicity is very low and won't have any detrimental effects on people or animals WEll unless they drink it I suppose.
The trick with such products is to consider life cycle. So if the mites have a 10 day life cycle sprayng every two weeks will not eradicate them.
So once you have picked your pesticide you now have to think about the most effective way to spray it. The best time to spray is at the end of the day. The other thing that is EXTREMELY important is to make sure the plants are well watered before you spray. Dry plants can be damaged by pesticides This is called Phytoxicity.
So my recomendations are; use something like End All, Trounce or Dr Doom These all contain Pyrethrins. Spray every 10 days for three weeks and time your spraying for the evening, sun and pesticides don't mix, and make sure the plants have been watered that day. The other thing you can do is mist the plants daily with cool water because spidermites HATE wet humid conditions and cold water so that will also help. Over the long term take a look at your cultural conditions and possibly not let your plants get so dry but even more important you need to raise the humidity to at least 60 % higher is better.
This should help in the bug fights!
Much happen here today.
I have never had this much trouble with a pest....if I didn't cherish the plants so much I would set the little buggers on fire!! 
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