Hmm, Orthene (acephate) is one of the most widely used pesticides for tobacco. That would take care of the larval stages. Just sprinkle a little (maybe 1/2 tsp per plant or less) on the soil. Water it in and it'll go in through the roots. You can buy it wherever you find ant poison, which is what it's most commonly sold as in Megalomart-type stores. I don't know if it has any phytotoxicity in weed -- I've only ever seen it damage a fern. There may be an interval required between application and harvest. Hmm, yes there is -- three to five days pre-harvest in tobacco. You could give it a week or more if you're the cautious sort. And I guess that if you water it into the soil, instead of using a foliar spray, it could perist even longer, which could be not good. Oh, and don't touch it. It is toxic to you until it breaks down. And you might want to not touch the plants for a while. Sounds like risky stuff, no?

Hmm, again. "Orthene (acephate) Do not apply this product through any type of irrigation system." Wonder why? Bad juju? Corrosive?

Idunno. Perhaps more experienced voices will warn you that acephate is pure death to you, your irrigation setup and your pretty green weed babies. Probably you should disregard everything I've written unless the head grow heads here tell you differently.