Two distinct species of fire ants (genus Solenopsis) from South America have been introduced into the United States this century. The South American black fire ant (S. richteri) was first introduced into the United States in 1918. Its close relative, the red fire ant (S. wagneri), was introduced in 1940, probably escaping from a South American freighter docked in Mobile, Alabama. Both species became established in the southeastern United States, spreading into nine states from Texas across to Florida and up into the Carolinas. It is estimated that they have infested over 320 million acres (130 million ha) covering 13 states as well as Puerto Rico.

Successful introduced species are often more aggressive than their native counterparts, and this is definitely true of fire ants. They are very small, averaging 0.2 in (5 mm) in length, but their aggressive, swarming behavior makes them a threat to livestock and pets as well as humans...