Cross Tolerance between Heat and Cold Stress by Warm Temperate Aquarius paludum paludum and Subtropical Aquarius paludum amamiensis, Semi-Aquatic Bugs (Gerridae, Heteroptera)

Abstract

The effects of geographical location (Kochi-strain [33°N]: Aquarius paludum paludum, Naha-strain [26°N]: A. p. amamiensis) and/or rearing temperature were examined on the lower and higher lethal temperatures (LLT and HLT) of larvae and also on the super-cooling point (SCP) of the adult water strider. Experiments to determine the lethal temperatures were performed mainly within 24-48 hours after the hatching or molting of the first to 5(superscript th) (last) instars. The first instars just after hatching were reared at 25℃ or 20℃ for the Kochi-strain and at 25℃ for the Naha-strain. The air temperature inside the incubating chamber was dropped or raised stepwise by 1℃ every hour to find the low and high lethal temperatures of the larvae on the water surface, respectively. The higher resistance to both lower and higher temperatures was shown by the first instars of the Naha-strain-25℃ group (mean HLT: 45.1℃; LLT: -3.38℃) followed by Kochi-strain-25℃ group (HLT: 42.6℃; LLT: -1.40℃). Similar differences in the lower and higher lethal temperatures were also shown by the 3(superscript rd) instars between the two strains. However, that difference disappeared by the 5(superscript th) (last) instar stage, probably due to exposure to a constant ambient temperature till the 5(superscript th) instar stage. On the other hand, in the Kochi-strain the 5(superscript th) instars grown at 20℃ showed both a higher mean HLT (45.34℃) and a lower LLT (-5.30℃) than those grown at 25℃ (HLT: 43.71℃, LLT: -3.20℃). The average SCPs in the A. p. amamiensis and Hybrid (F15) were significantly lower than those of A. p. paludum under long- and short-day photoperiods. Natural selection of heat tolerance in the Naha subtropical strain might also lead to an accompanying higher cold hardiness. The overwintering generation of larvae grow up in relatively low ambient temperature of around 20℃ during late October and November in Kochi. This moderate temperature could lead to higher cold hardiness of the last instars just before winter.