How tolerant are ants to cold?
Acclimatization increases cold resistance of ants [1]. They have a strong adaptability to different environments [2].

When it comes to the red imported fire ant (Solenopsis invicta) here are some numbers (from [2]):


  When the exposure time was 0.5 h, the survival rate of workers was 15.3% at -14°C. At the exposure time of 1, 2 and 4 h, the entire lethal temperatures for S. invicta were -13°C, -9°C and -9°C, respectively. When the exposure temperature was -10°C, all workers died within 240 min; while at -11°C, it took only 120 min.




References:


Modlmeier AP, Pamminger T, Foitzik S, Scharf I. Cold resistance depends on acclimation and behavioral caste in a temperate ant. Naturwissenschaften. 2012 Oct;99(10):811-9. doi: 10.1007/s00114-012-0963-8. PubMed PMID: 22955370. 
Xu YiJuan; Lu YongYue; Huang Jun; Zeng Ling; Liang GuangWen. Cold hardiness of natural populations of the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae) in Shenzhen, Guangdong. Acta Entomologica Sinica 2009 Vol. 52 No. 9 pp. 974-983.