Humidity preference and temperature tolerances of four species of fire ants (Hymenoptera: Formicidae: Solenopsis)

Distributional data of fire ants in Texas indicated differences in temperature and moisture requirements, A study was begun to more fully understand the effects of these two factors and to determine if laboratory responses would correlate with distributional differences. Four fire ant species found in Texas, Solenopsis aurea, S, geminata, S.invicta, and S. xyloni were used in tests of humidity preference, supercooling temperatures, and temperature tolerances. Attempts to acclimatize the ants failed. Humidity preferences were determined with a linear gradient chamber. The four species showed a marked preference for saturated atmosphere {near 100* RH). Supercooling points were determined with a hypodermic needle microprobe and tel-etherraometer- The mean supercooling points for the ants were -6.7° C for S. aurea, -8.6• C for S. geminata, -8.3• C for S. invicta, and -7.0° C for S. xyloni: Solenopsis aurea and S- xyloni ware not significantly different from each other, and S, invicta and S, geminata were not significantly different from each other; the two groups were significantly different. Three caste sizes were tested in the supercooling experiments. There were no differences between the castes of S. aurea and S. xyloni.- Solenopsis invicta major workers supercooled at a significantly lower temperature than did the medium and minor workers, Solenopsis geminate minor workers supercooled at a significantly lower temperature than did the other castes. Temperature tolerances were determined from one hour exposures. Temperature became a significant cause of mortality at approximately 40» C, and at 450 C all ants perished- Regression analysis showed high variability within replications and significance of the independent effects of temperature, species, humidity, and interactions of these effects. The significance of the independent effects was partly due to the confounding of these effects by the variability within replications. All LD50 values were between 41° C and 44°C.