Abstract: To obtain a comprehensive evaluation of cold hardiness of the red imported fire ant, Solenopsis invicta Buren, the super cooling points (SCPs) of different casts at developmental stages and the ability of workers to survive at low temperature were measured based on materials collected from S. invicta natural populations of Shenzhen, Guangdong in 2005-2006. The results showed that there was a significant difference in SCPs between casts and three developmental stages while the SCPs of the larva, adult and pupa reduced successively. No significant difference was found between the workers and the winged ants in SCPs, but significant difference existed between male and female winged ants. SCPs fluctuated over winter, reduced from October and got to the bottom (-12.68℃) in February, and rose to -9.51℃ in March. The survivorship of workers (S^T) showed a strong relationship with the exposure temperature (T) and the exposure time (t), fitting the Logistic equation. When the exposure time was 0.5 h, the survival rate of workers was 15.3％ at -14℃. At the exposure time of 1, 2 and 4 h, the entire lethal temperatures for S. invicta were -13℃, -9℃ and -9℃, respectively. When the exposure temperature was -10℃, all workers died within 240 min; while at -11℃, it took only 120 min. Fluctuation and persistence of low-temperature were correlative with the survival rate of workers. Under more fluctuating low temperature, the workers had a higher mortality. It took 9 d for the survival rate to drop to 0 under the condition of 4±9℃, while it was longer than 15 d for that under 4±0℃. But the survivors grew stronger in cold tolerance. Compared with the control, treatments under fluctuating temperature of 4±6℃ and 4±9℃ for 4 d could raise the survived workers’ cold tolerance at low temperature significantly. The results suggest that the fire ant can obtain rapid acclimatization when exposed to low temperatures, and it has strong adaptability to extreme weather.

Key words: Solenopsis invicta, natural population, cold hardiness, super cooling point (SCP), low temperature, acclimatization