In the last couple of months, I have continually found these tiny black bugs (literally the size of a pencil tip) on the trays beneath my pots. They start moving as soon as I lift the pots. I have inspected the plants (haworthias and jades) and never found any bugs on the actual plants. I tried using imidaclorid spikes but they haven't made a difference. I wipe up the bugs when I see them, but within a week they are back. For reference, my plants are in a potting medium of granite chips and turface.

Any ideas on what they might be? Thanks!