They do spend part of their life in the soil. I think they have 4 growth stages, at one of them, they will drop to the soil and transform into their next form. I've found spinosad will kill them on contact and it can be used into flower. If you alternate it with bt (another bacteria that kills them), you can probably knock them down in plenty of time to harvest. Every 2 days, drench leaves and soil with one, then alternate to the other.