In the United States, the red imported fire ant first arrived in the seaport of Mobile, Alabama, by cargo ship between 1933 and 1945. Arriving with an estimated 9 to 20 unrelated queens, the red imported fire ant was only rare at the time, as entomologists were unable to collect any specimens (with the earliest observations first made in 1942, preceded by a population expansion in 1937); the population of these ants exploded by the 1950s. Since its introduction to the United States, the red imported fire ant has spread throughout the southern states and northeastern Mexico, negatively affecting wildlife and causing economic damage. The expansion of red imported fire ants may be limited since they are almost wiped out during Tennessee winters, thus they may be reaching their northernmost range. However, global warming may allow the red imported fire ant to expand its geographical range. As of 2004, the ant is found in 13 states and occupies over 128 million hectares of land, and as many as 400 mounds can be found on a single acre of land. The United States Department of Agriculture estimates that they expand westward per year. Likely due to absence of South American competitors - and lower numbers of native competitors - "S. invicta" dominates more extrafloral nectaries and hemipteran honeydew sources in the southern USA than in its home range.