I don't have an answer, though it looks more and more like my mint pest is thrips. From what I've read both the soil and plant need to be treated. The eggs are laid in the plant where hatched thrips emerges and feeds on the leaves and stem before falling into the soil where it stays until it metamorphosizes into its adult winged self (then repeat cycle by laying eggs in the plant). In the meantime you might want to put up some blue sticky traps to attract and capture the adult thrips.

Hopefully, someone with experience will answer.