On the lawn next to the sidewalk a fire ant colony is swarming. The ants are pouring out of a mound nest, here no more than an irregular pile of dirt partly flattened by the last pass of a lawnmower. Winged queens and males are taking off on their nuptial flight, protected by angry-looking workers that run up and down the grass blades and out onto the blistering-hot concrete of the sidewalk. The species is unmistakably Solenopsis geminata, the native fire ant.