Spider mites on cannabis plants
What it looks like
Tiny pale specks on the leaf surface, then fine webbing under leaves and at the tops. You can see the mites move on the underside. They spread fast in a hot, dry room.
What usually causes it
Isolate the plant. Wipe or remove the worst leaves. Lower the heat and raise humidity a little so the room is less friendly to them. This page does not recommend a spray. If you use a product, it has to be one that is legal where you grow, and you follow that label.
Check this first
Fine stippling on the top of the leaf, tiny movers on the underside, webbing if it is bad. Look under the leaf with a light. A deficiency does not crawl.
This is identification for a home grow. It is not a pesticide label and not medical advice. Follow your local law.