Callcott and Collins reviewed the spread of the red imported fire ant, Solenopsis invicta, from 1939 to 1995 with a series of maps.* The infestation started in Mobile, AL and by 1939 had spread to the surrounding area. The pattern of spread is a slowly expanding circle until 1965. Then the northern expansion slows, as expansion along an east-west axis and to the south continues. Fire ant reproductives can fly short distances to establish new colonies and account for much of the local spread. However, appearance of new colonies miles from the contiguous range are indicative of human assisted spread. Since their survey in 1995, some colonies appeared on the west coast and in Kansas that were attributed to the movement of nursery plants.
Fire ants can colonize potted plants kept outdoors and be moved hundreds of miles by owners. Fire ants are easily overlooked if the colony is small. Exact identification requires experts who evaluated subtle morphological features only visible by microscope. Colonies often escape attention until reports of people stung by aggressive ants are investigated by entomologists. It was thought that the northern expansion of fire ants was limited by cold winter temperatures. However, Kansas entomologists found that populations can overwinter in Kansas and are only eradicated with aggressive treatment.
Since 1995, the red imported fire ant has moved north along the east coast into Virginia and Maryland, although those populations are sparse and subject to eradication. Fire ants have appeared on the west coast in spite of the quarantine against moving plants and soil. It is likely that people uneducated about the problem are not in compliance and are assisting the further spread of this pest.
*ANNE-MARIE A. CALLCOTT AND HOMER L. COLLINS. 1996. INVASION AND RANGE EXPANSION OF IMPORTED FIRE ANTS IN NORTH AMERICA FROM 1918-1995.Florida Entomologist 79:240-251.