Chunky Stool
Plant Destroyer

I'm getting ready to harvest a crop and noticed that there are mites in the grow room!

These aren't spider webs, and this area was completely free of webbing when I started.
There were plants from last summer that I brought in from outdoors (mistake) and they had mites.
Thought I had cleaned thoroughly and killed them all... but obviously a few survived.
So here's the big question... why don't my plants have mites? I pulled a few out and checked every nook & cranny with a light & magnification.
I'm thinking there might be two reasons:
1) My plants are healthy. They are "semi-organic" and have always been vigorous.
2) I sprayed with a sulfur & pyrethrin product a week before starting 12/12, and it left a residue on the leaves.

I'm not complaining, but it seems odd.
What do you think?