Physiology of Overwintering in a Microhabitat Fully Exposed to Adverse Weather Conditions: Lime Seed Bugs on Tree Trunks and Branches.

The present study investigates the physiological aspects of overwintering in an exposed microhabitat in the lime seed bug, Oxycarenus lavaterae. We found that the overwintering lime seed bugs do not survive freezing of their body fluids, but instead rely on supercooling (freeze avoidance). The seasonal modulation of the supercooling capacity was very limited, with the midwinter mean supercooling point reaching -15.5°C, but the individual variability was very high (- 6°C to - 22°C). Most of the other physiological parameters of overwintering lime seed bugs (utilization of energy substrates, changes in hydration, and metabolite composition [although metabolite levels were low]) were consistent with the general knowledge gathered for other freeze-avoiding insects. A significant exception was found in the amount of osmotically active water ("freezable" water), which constituted up to 95% of the lime seed bug body water. Such a proportion is unusually high, as it typically ranges from 59% to 86% in other insects and invertebrates. At present, we have no plausible explanation for this anomaly or its possible relationship to the lime seed bug's overwintering microhabitat.