As I recall, for my location, they start hatching in mid-May, and maybe hatch for 2-3 weeks?

Grow as larva thru June into early July.  In mid-to-late July, they pupate, and emerge as adults. Shortly after - in mid-August, they are laying eggs.  The fact that you got your volunteer corn killed in mid-to-late August tells me those rootworms had corn roots to live on, long enough to get to the adult stage.  You really need to get that vol corn killed in May, in order to break that life cycle for the rootworms.  Maybe killing vol corn in June is soon enough. Killing vol corn in July is too late to break the rootworm cycle. In my humble opinion.