Leg
Your proposal of chemical is effectively an option especially when you know the amount of chemicals you'll find in most farms. I'm not a specialist but from what I recall most (if not all) combat gas were derived from farm chemicals? However, the farmers might want to keep it for later growing season.
Another problem will be the use of these products. A year ago someone came to my house in order to treat an infestation (full NBC suit; I was laughing my ass off). When he finished he took out the suit, open the staircase door and overlooked the fact that a good part of the product would rush into that staircase. I started to smell it (I was staying one floor under him) and about a minute later you had two green guys trying to catch their breath at the front door
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It's a good idea but you might kill many of the defenders along with the assaulters. Actually, that could be interesting in term of game managing.
