Supercooling of the red imported fire ant along a longitudinal temperature gradient in Texas

Most insects are freeze intolerant and employ supercooling as a mechanism of survival. By supercooling, or physiologically lowering the point at which they freeze, insects withstand subzero temperatures. Since its introduction into the southern United States, the red imported fire ant, Solenopsis invicta Buren, a freeze intolerant species, has extended its range north and now occupies areas where winter temperatures are consistently below freezing. Therefore, the objective of this study was to determine if S. invicta of northern Texas has increased supercooling ability compared to S. invicta of southern Texas.

Colonies were collected in early and late summer and early spring from Lubbock, Kerrville, and Brownsville, Texas, and acclimated in the laboratory for two months. Analysis of variance and Fisher's LSD test indicated significant differences (P < 0.05) in supercooling ability of populations among location. Surprisingly, colonies from northern Texas (Lubbock: -4 to -6- C) were less able to supercool than colonies from southern Texas (Brownsville: -10 to -17- C). As expected, colonies from central Texas showed intermediate supercooling ability (Kerrville: -3 to -11- C) between the two extremes. In addition, differences (P < 0.05) were found among all seasons for each location, a pattern typical of other freeze intolerant species. The supercooling ability of S. invicta may therefore be a "remnant" ability, not yet reflecting its survival needs as suggested by severe winter temperatures.