A friend noticed mites in one of my Grammostola enclosures. Substrate was still moist due to a recent rehouse. Here's what I did:

1. I took a bottlecap that would fit in the corner of the enclosure and cleaned it
2. Placed it inside the enclosure and filled it with water.
3. DO NOT add water onto the substrate.
4. Checked the day after and found all the mites drowned in the water pool.

Here's my observation. Mites love moisture and a humid environment. They will look for water sources and so if they cannot get water from the substrate, they will run into the only place where water is available -- yes, the bottle cap. There they will drown and die.