Spider mites keep me awake at night. These evil little bastards can float in on a puff of air, a piece of clothing, your dog, your girlfriend, or a bad idea. I hate them, and they hate me back. They are nearly impossible to eradicate. I take that back; they are impossible to eradicate. The only way to beat them is to stop all production and go get the 10-percent bleach solution. Then get all new plants and hope for the best.
You may not be able to eradicate them, but you can control them. And, depending on how serious you are about it, you can do a fairly okay job of it. In Colorado, there are two kinds of professional grow ops: those that are fighting spider mites and those that are lying about fighting spider mites.
My home-grow, peace be upon it, still has no spider mites as of this writing. (Still! In 2014!) That is probably because I refuse to enter my grow space unless I have recently showered, shampooed and changed my clothing and my girlfriend. The dogs are not allowed in, nor are bad ideas. (I was kidding about my GF, by the way!) I also never introduce a new strain unless it has been quarantined in a separate space for at least a week.
When it comes to controlling spider mite infestations, I should start by saying what does not work–at all. No matter how many good vibes and rainbows you put in the jar, home remedies involving garlic, oils, dish detergents, tobacco, or anything else in this category are simply not effective. Whenever I read posts touting these remedies on the forums, I think, “Here’s someone who is offering advice that has not been tried on a spider mite infestation that existed within the scope of reality.” Believe me; I have fought a lot of mite infestations. These critters are Tough with a capital T.
So, what does work? It’s a short list. If the plants are not flowering, and they are still very small, destroy them, bleach and start fresh. If the plants are not flowering, but they are big, you will have to go for the evil stuff–the newest horror-show-in-a-bottle–and hammer those little vampires every 48 hours, because they have a short life-cycle (about three days).
A product I have used with good success, especially during flowering, is Hot Shot(R) No-Pest(R) Strips. This product is sold in most hardware stores, and its active ingredient is the chemical dichlorvos. This chemical is also used in the treatment of livestock feed to prevent insect infestation. The No-Pest Strips are not very effective in commercial situations–it's just not possible to use enough of them in such a large space–but for a 4' x 4' or 4' x 8' tent or closet, two of them will set your mites back without you having to apply anything directly to your plants. Their toxicity is high; they cause cancer; they are officially Not Good; but they work. We're all adults here; you make the call.
Also, please try to avoid using neonicotinoids. Yes, they work; however, these compounds kill bees. If you are growing inside, you should consider using them if, and only if, you can dispose of the toxified plant material safely and without exposing bees to it. If you are growing outside, you should not use neonicotinoids. If you are uncertain about a particular chemical's toxicity, you can check online at http://www.pesticideinfo.org.
Another method of controlling spider mites is to tent them up and gas them with CO₂ at 15,000 ppm for a few hours. Be aware that you can kill plants, pets and people this way too, so be careful. Do this every three days until you have given the infested area four gassings. Then, once you have knocked the spider mites back, you can attempt to control them until flowering using more tame stuff like Beethoven(R) or AzaMax(R) and vacuum cleaner nozzles. Once the plants start flowering, you do not want this stuff on your bud. At all.
This bears repeating: Once the plants are flowering, no chemical, no matter how “safe,” should ever be used. Never forget that you will be inhaling this plant eventually. Predators should not be used either, because this method simply trades one insect problem for another. I have seen modest success on modest infestations with some of the live mite predators that are available. The problem is, the mites always come roaring back; and if you are flowering, you will then find yourself with mites and predators sticking to your bud.
Plants infested with spider mites in late flowering must be harvested for use in extraction products. This effectively separates the pest from the useful components. No other use for a plant so infested is acceptable in professional settings; use your own judgment when growing pot for your own personal use. Spider mites often crackle and pop when you smoke them, which is not pleasant.
If you are in late flowering, I do have one trick to offer you. You can outsmart the little shits. Firmly attach your plant to a stake. If the stake is the highest point, many of the mites will climb up to the top of the stake within a few hours. Once there, they can be wiped off with a wet paper towel. I noticed this by accident one day while working in a badly-infested medicinal grow. Note: this is not a cure; rather, it is a way to reduce their numbers before harvesting for extraction. Good luck–because you’re never done with spider mites.
If you are finding this information useful, why not pick up a copy of Marijuana Cultivation Reconsidered? It contains this information, plus so much more. Marijuana Cultivation Reconsidered is over 300 pages, with over 90 images and illustrations. Danny Danko describes it as “meticulously researched and enormously useful. . . a must-read for any grower striving to learn more than the basics and think outside the parameters of ‘conventional wisdom’ and horticulture folklore.”