I see many, many threads on spider mite infestations.
Spider mites live in the environment naturally. The type of mite that bothers us is most prevalent when temperatures stay above 50 degrees. You may see the nests in the grass, in tree branches, on flowers, etc. Spider mites live off the juices that the plant produces which are essentially sugars. The prefer to eat on the bottom side of the leave as it is easier to access the succulent parts of the leaf.
They also lay their eggs on the bottom side of the leaf. Incubation is 7-14 days, which is an important fact. Control of mites if an infestation occurs, centers more around stopping the hatchlings as few non-toxic's work on mites.
An infestation can ruin an entire crop or just one plant, depending. Not worth the risk of letting an infestation occur in the first place. If a bad infestation occurs, i.e., webs on the buds, remove the plant as gently as possible so as to not loose any mites on the way out of the grow room. Then destroy the plant and the mites and discard. (The eggs will still hatch). Watch other plants for signs...small dots on most of the leaves. They are only 1/50th of an inch long so they are very hard to see. Remove all the plants and treat as below and sterilize the grow area with a bleach/water solution and allow to dry before putting plants back in.
Prevention is the best solution to mites. Seven(brand name) seems to be a great perimeter poison to use around the exterior of the grow area and into the air duct openings (while off). Cracks and crevices in the room itself also should get a barrier stream sprayed down for the same reason. Do this about once a month during mite season and occasionally in off season as it also controls other bugs.
What about inside the grow area? Some folks like to keep the Shell No-Pest Strip in the grow area. Not sure if plants uptake this or not, so will not address the use of it, but it is commonly used. Neem oil is a product of the Neem tree. Natural as natural can be. The only effect on us the consumer, is if used too late it may affect the taste. No issue to us even if smoked.
Neem can be applied multiple ways. It can be given in the water and the plant will uptake enough to provide the barrier. Myself I prefer to just use foliar spray in the vegitative room only. (My air path is such the flower room is the most protected and room the air is evacuated from, veg room vents to the flower room. During the season I foliar spray the veg plants as soon as they are rooted about every other day. I watch the leaves. Shiny leaves have enough Neem for their life and need no more.
We do not want to use so much we choke off the stoma of the plants, so do watch the leaves. Allow the spray to dry before repositioning the lights close to the leaves. Some species are less tolerant of Neem, but all do very well with this process.
Why Neem? Because it prevents the offspring/hatchlings from being able to digest food and it kills them before they can mature and make more eggs. It is critical to understand that the hatchlings must be controlled or the infestation will continue and you will have consistent issues with mites.
If anyone has a good shot of an infestation with web/nest, it sure would be nice of you to post that to go with this thread.
Link with good knowledge:
The Life Cycle of Spider Mites | eHow.com