Caste and sex differences in cold-hardiness in the social wasps, Polistes annularis and P. exclamans (Hymenoptera: Vespidae)

Elevated fructose, glucose and trehalose levels were found in Polistes exclamans and P. annularis after exposure to cold. Glycerol was found only after extensive exposure to cold or in mid-winter. In P. exclamans mean supercooling points (SCP) decreased to a low of -12.9° C in February. In P. annularis SCP were never below -6.8° C in the field, but laboratory acclimation at +5° C resulted in SCP of -10.7° C. Freezing was lethal to adult Polistes under all circumstances. Workers and future queens were separated on the basis of the appearance of their fat bodies. For P. exclamans, only 17 % of the future queens died during 15 days at +5° C (N=29) while 76 % of the workers died during identical cold exposure (N=45). The surviving future queens had elevated cryoprotectant levels consisting mainly of fructose in their hemolymph, while the few workers that survived did not. These experiments indicate that there are physiological differences between workers and future queens in Polistes collected in autumn and that fat body appearance determined using non-invasive techniques is a reliable indicator of caste. Males were similar to future queens in their response to cold indicating that autumn mating activity may extend into winter.