Spider Mites!
Thank you for all the assistance from your books and articles in HIGH TIMES that have helped me grow medical marijuana under California’s Proposition 215.

I have a 6’ x 8’ outdoor shed that I divided into flowering and vegetative rooms with a 400w HPS lamp in each. There is an exhaust fan, a Green Air Products propane CO2 generator and the room stays between 80°-85°. My first crop was beautiful, but the last two have been inundated by spider mites. I tried BANG (pepper-based insecticidal soap spray) and a solution of isopropyl alcohol and water. The plants are looking better, but the foliage is damaged and buds are suffering. Even after cleaning the entire space with bleach and repainting, the mites are getting back through the intake duct.
Grimace
Bay Area
California

Dear Grimace,
Spider mites are worst in climates such as yours that seldom freeze. A freeze kills most eggs outdoors, and dry air from a heat system keeps their reproductive cycle slowed down. Your attention to cleanliness must continue. Put a very small mesh silkscreen over the intake, and wet it down regularly with insecticidal soap. Mites have a very difficult time passing through a miticide-treated wet silkscreen. Smear Tanglefoot (a sticky trap gel) around other possible unsealed small entrances. Spray all around the shed with a strong miticide, to lay out a demilitarized zone. The all-out war starts inside for any mite that is unlucky enough to enter.

Always dip clones in miticide before moving them into the flowering room. There are quite a few miticides available that work well. Two of the best are aerosol pyrethrum and neem oil. My favorite brand of neem is Einstein Oil. The trick to killing spider mites is to make sure the spray touches them. Tilt plants up on their side and spray leaf undersides where mites and their eggs reside. Miticides approved for food crops must come in contact with mites and eggs in order to kill them. If spray only touches the top of foliage, mites are not affected.

Keep the entire room clean at all times, so there is nowhere for mites to hide. Inspect for mites daily and spot spray anywhere you find them. Eggs will continue to hatch for weeks, and new mites must be killed before they get a chance to lay more. Keep up a rigorous eradication for at least 6 months, and you should have the problem well under control.