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You're all going to love this. It's environmentally friendly, it's inexpensive, it's 100% lethal, and makes them die in the most horrible manner possible.
Dish soap.
Put a tablespoon or two into a spray bottle, then add about 2-3 cups of water. Swish the water to mix. Do not shake! You don't want foam in the bottle.
MOVE. UP. STREAM. Seriously, you don't want to be downstream of this when it happens.
Now, starting in the middle of the mass, start spritzing with the soapy water, and work your way out to the edges.
The soap will break the surface tension, and act as a wetting agent on the ants. As a result, the water will now start wicking up their bodies, enveloping them like living quicksand. And with the surface tension broken, the mass of ants will start to sink.
Remember, start in the middle! This will cause the center to sink first, and drag the edges down last. The ants will all hold on to each other and drag every last one below the surface. With a soft bloop, the entire mass will drop.
Remember how I said to move upstream? If you're downstream, the entire submerged mass could end up wrapped around your legs. Where they will immediately begin clawing their way to the surface, using you as a ladder.
Best of all, you can scale up your operation, and load a backpack sprayer with a bottle of Dawn and a few gallons of water. Then just wade through your neighborhood, wiping them out for the benefit of everyone!
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[1] https://www.reddit.com/r/natureismetal/comments/6wkd4j/firea...
[2] https://www.livescience.com/13867-raft-fire-ants-buoyancy-fl...