Submitted to: Population Ecology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/6/2001
Publication Date: 11/7/2002
Citation: JAMES, S.S., PEREIRA, R.M., VAIL, K.M., OWNLEY, B.H. SURVIVAL OF IMPORTED FIRE ANT (HYMENOPTERA: FORMICIDAE) SPECIES SUBJECTED TO FREEZING AND NEAR-FREEZING TEMPERATURES. POPULATION ECOLOGY. 2002. v. 31(1). p. 127-133. Interpretive Summary: Imported fire ants have been spreading northward in the U. S. since its introduction early on the 20th century. Because these ants are from tropical and subtropical regions, they were not expected to survive extreme cold weather. However, the imported fire ants have expanded their range and now occupy regions where the average minimum January temperature is below 10 degrees F. Understanding the cold-survival in fire ant population is important for the determination of areas that are at risk of fire ant infestation. Also, the ability to survive cold temperatures may be influenced by biological control agents being introduced in the U. S. Scientists at USDA-ARS, CMAVE, Gainesville, FL and the Dept. of Entomology and Plant Pathology, Univ. of Tennessee, Knoxville, TN studied the low temperature survival of the red imported fire ant, the black imported fire ant, and their hybrid. They also examined the effects of seasonal acclimation and infection with the protozoan T. solenopsae on imported fire ant tolerance to low temperatures. The lowest temperatures the ants can withstand before freezing was not a good measure of the cold-tolerance of fire ants. However, long-term exposure to low temperatures showed that hybrid fire ants are more cold-tolerant than the parent species. Also, red imported fire ants infected with the protozoan were more tolerant of cold temperatures than uninfected ants. Northern range expansion favors the hybrid over either of its parents.
Technical Abstract: 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, and Hazard (Microsporida: Thelohaniidae) infection on these parameters, were examined. .We tested Solenopsis richteri Forel (Hymenoptera: Formicidae), S. richter 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 degrees 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 degrees 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 US.