SUPERCOOLING OF THE RED IMPORTED FIRE ANT (HYMENOPTERA: FORMICIDAE) ON A LATITUDINAL TEMPERATURE GRADIENT IN TEXAS

Abstract

Since its introduction into the United States, the imported red fire ant (Solenopsis invicta Buren), a freeze-intolerant species, has extended its range northward and now occupies areas where winter temperatures are consistently below freezing. The objective of our study was to determine if more northern S. invicta had greater supercooling capability than more southerly S. invicta. Such information would help to delineate the northern boundary of range expansion. We collected S. invicta colonies in early summer, late summer, and early spring from 3 locations along a latitudinal gradient in Texas. Supercooling point determinations were made immediately after colonies were collected and after colonies were acclimated in the laboratory for 1 and 2 months. Regardless of time or acclimation period, the northernmost colonies (−5.91 ± 0.21°C) had higher mean supercooling points than the southernmost colonies (−16.49 ± 0.11°C). Colonies from central Texas had intermediate mean supercooling points (−10.40 ± 0.19°C). This pattern is difficult to explain. Tests from a broader geographic range might allow us to better assess supercooling ability in S. invicta. Additionally, we tested whether supercooling points were affected by body size. We found no relationships between supercooling points and head capsule widths. However, there was little variation in head capsule widths of the minor workers that we measured. Differences in supercooling points might be evident only on a larger scale of body size.