That's an interesting question and one I don't have a definitive answer for. But thinking about the life cycle of soil mites and how they live in the soil , I doubt BTI would have any effect on them. Since BTI is a bacteria found in soil the soil mites are likely not affected by BTI. It works on fungus gnats and many other pests but since soil mites are basically part of the soil ecosystem they can probably live in harmony with that bacteria.
I'm not a soil microbiologist but I'm going to text my niece and ask.
OK, she responded right away. They are not bad for the plant. They assist with breaking down the soil in a similar fashion as earthworms on a smaller scale. BTI will not have any negative effect on them since they live in the soil where bacillus thuringiensis lives.
"They're presence is correlated with soil pH and vegetative cover. They scavenge organic matter, so they're important for decomposition. Probably means you have a good amount of organic matter in your soil."
That's almost verbatim. She's working on her PhD. and knows what she's talking about. Although she did say that they are intermediate hosts for tapeworms. I wouldn't worry about that at all. You won't get tapeworms from soil unless you eat it.