dry ice to kill ground squirrels?

I have a terrible ground squirrel problem. They're tearing up everything. I've fed them incredible amounts of the commercial squirrel bait for 2 months with little effect, tried live catch traps, conibear traps and rat traps with 0 success. (which was hard to believe, but they wouldn't go near any of them.) Tried shooting and after a day of effort I got 1 and maybe wounded another. They keep expanding. There are at least 100 holes now. It's bizarre, almost supernatural. I thought about the rodent blaster thing with flammable gas but there are trees and dry grass everywhere so the fire risk is just too high. I'm out of ideas.

The thought came to mind of putting dry ice down the holes to asphyxiate them with co2. I've found some suggestion of it on the net and it seems like it would work, but I haven't come across anyone who actually tried it.

Yes? No? Practical or realistic ideas anyone? I'm at my wits end, these things have caused thousands of $ in damage and they must be stopped.