Come on, it's been years since I've tried very hard to not think about this detail.
Also, if you want to keep the planet clean and bioweapons are too much of a hassle to fine-tune, why wouldn't they use crude but massive ships to jump in-jump out at low altitude? They clearly showed the ability for double-jumps in the first (or second) episode with a raider.
That's why when Niven and Pournelle worked their drive out for Mote in God's Eye, they went to physicist Dan Alderson and carefully weighted the implications of varied types of drives.
This one is actually the worst possible combination in regard of not wiping civilisation up in the next five minutes.