Welcome to NGA!
Could you go back and update your info to include where you are and what zone you are in? It will help us to help you.
Without knowing what the insects are, I would think some sort of systemic like Bonide, which is granular - dig it into the top couple inches of the soil. Or a spray on systemic like Bayer 3 in 1.
Is it possible to take a picture of these little black crawlies?