I live on the NC Piedmont. My yard gets tiny red ants with low mounds
about a foot in diameter. They swarm when disturbed. I understand it
takes an expert to see the difference between native fire ants and those
from Brazil.

I understand a 4% mixture of borax in sugar will kill fire ants if the
syrup is thin enough for them to suck. When I put some in a lid on the
mound, I discovered big red ants, that probably weigh ten times more
than the little ones, peacefully among the fire ants on their mound.

I thought fire ants didn't tolerate other ants. Was I wrong?