Survival at low temperatures is an important parameter determining distribution of imported fire ants in the United States. Supercooling points and survival at low temperatures, and the effects of species, individual size, and Thelohania solenopsae Knell, Allen & Hazard (Microsporida: Thelohaniidae) infection on these parameters, were examined. We tested Solenopsis richteri Forel, S. richteri X invicta hybrid, and Solenopsis invicta Buren. Great variation was observed in the supercooling points, which are not an appropriate measure of cold hardiness for imported fire ants. When exposed to near-freezing temperatures above their supercooling points, fire ants died at different rates depending on the species and T. solenopsae-infection status. Extended exposure to 4°C resulted in both the hybrid and S. invicta infected with T. solenopsae having significantly lower mortality rates than either the S. richteri or the uninfected S. invicta. At 0.5°C, the hybrids had significantly lower mortality than the uninfected S. invicta, but mortalities for S. richteri and T. solenopsae-infected S. invicta were not significantly different from each other or the hybrid. Ant mortality was 100% for all ant types after 7 d at −4°C. The uninfected S. invicta was consistently less cold-tolerant than the other ant types. The hybrid fire ants and the T. solenopsae-infected S. invicta had the lowest mortalities. These results support the hypothesis that extended cold injury causes winter kill of fire ants, and may partially explain the distribution of fire ant species in the United States.
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