You have to keep hitting them to break up the reproductive cycle. When spider mites are about to die, they lay eggs, so just when it looks like they are all gone, they will reappear. They are evil bastards and need to be crushed. Do you have an 8 week strain? If it's a 6 week strain on week for, that makes a difference.
First step is isolate any infected plants, if there are any not affected ones left. If you have 4 weeks left, you're probably going to want two (some recommend 3)
pyrethrin bombs to kill all the living ones, their offspring, and any stragglers. The reason you need two or three is that rebound effect. Sprays are always phytotoxic, so it's possible you're going to burn the leaves a bit. This is tough to avoid at this stage of the game.
Azamax might work, but if you've got it bad, it's going to be tough, because most of the time it requires contact to stop them. Consider removing infected leaves, check EVERY leaf for eggs, and crush them by hand if there are only a few (doesn't sound like this is the case). Some mites are spray resistant, so you may need to rotate sprays.
Floramite or
avid might be a good step, I've heard good things about them, though fortunately have never had to use them (knock on wood). I keep a
pyrethrin bomb around at all times, just in case.
If you can get them now, predatory mites could be helpful, but you don't have a lot of time because of the rapid reproductive cycle. If you can drop the temperature in your grow space for a while, that should help slow the reproductive cycle. The hotter it is, the faster it goes. Here is a scale for mite damage that I just found on an ag website:
0 – No spider mites or injury observed.
1 – Minor stipling on lower leaves, no premature yellowing observed
2 – Stipling common on lower leaves, small areas or scattered plants with yellowing
3 – Heavy stipling on lower leaves with some stipling progressing into middle canopy. Mites present in middle canopy with scattered colonies in upper canopy. Lower leaf yellowing common. Small areas with lower leaf loss. (Spray Threshold)
4 – Lower leaf yellowing readily apparent. Leaf drop common. Stipling, webbing and mites common in middle canopy. Mites and minor stipling present in upper canopy. (Economic Loss)
5 – Lower leaf loss common, yellowing or browning moving up plant into middle canopy, stipling and distortion of upper leaves common. Mites present in high levels in middle and lower canopy.
In other words, if it's 4 or above, destroy the plant (maybe make hash if you're inclined, but there is a risk of the mites migrating, so you will want something like tanglefoot on the drying lines to stop them), bomb the grow room, and sterilize it.
