Supercooling studies on North American fire ants (Hymenoptera: Formicidae)

Supercooling points were determined for worker larvae, worker pupae, minor workers, medium workers, major workers, reproductive caste larvae, male and female pupae, and male and female adults of Solenopsis aurea, S. geminata, S. invicta, S. richteri and S. xyloni. The worker caste had a slightly lower supercooling point than the reproductive caste. Within each species adults had a higher mean supercooling temperature (-7° C to -12° C) than immatures, and larvae had a higher supercooling temperature (-8° C to -16° C) than the pupae (-16° C to -24° C). Nest construction appears to be more significant in explaining the northward distribution of fire ants in North America than do their limited physiological adaptations toward coldhardiness. The three species which build mounds riddled with galleries (S. geminata, S. invicta, S. richteri) rarely occur north of the -12.2° C January isotherm; whereas the two species without such "drafty" mounds (S. aurea, S. xyloni) occur north of the -20.6° C January isotherm.