TEMPERATURE REGULATION OF SUPERCOOLING AND GUT NUCLEATION IN RELATIONTO DIAPAUSE OF PYRRHOCORIS-APTERUS (L) (HETEROPTERA)

The heteropteran Pyrrhocoris apterus (L.) does not survive freezing of its body fluids; there is a good correlation between values of surviv al at subzero temperatures and the supercooling point (SCP), i.e., the temperature at which body fluids start to freeze. The decrease of the SCP and thus the increase in cold hardiness is regulated by photoperi od and temperature. The relative importance of these factors depends o n the physiological state of the insect. The SCP is about -7 degrees C at the onset of prediapause and a decrease of about 4-5 degrees C is associated with the development of the diapause syndrome in adults; th ese processes both are induced by a short-day photoperiod with tempera ture playing a secondary role. The induction of the diapause syndrome is a prerequisite for the subsequent decrease of the SCP by about 5-6 degrees C during cold acclimation. An intermediate temperature of 15 d egrees C, or fluctuating outdoor temperatures and short-day photoperio ds, are more suitable for the decrease of SCP than 5 degrees C in cont inuous darkness. The sensitivity to photoperiod gradually disappears d uring the development of diapause; after the termination of diapause a round the winter solstice the SCP irreversibly increases at a high tem perature af 26 degrees C even if exposed to a short-day photoperiod. T he SCPs of hemolymph, gut, fat body, and gonads were compared to whole -body SCP. The gut was identified as the primary site of ice nucleatio n because its SCP value was very similar to the value for the whole bo dy in both short-day and long-day insects. The SCPs of other organs, i ncluding the hemolymph, were always lower than the whole body SCP. Foo d was not a source of ice nucleating agents because the SCP of freshly ecdysed adults remained high after 2 weeks of starvation In contrast, feeding was a prerequisite for the decrease of the SCP during prediap ause. In postdiapause insects, the SCP increased at high temperatures in spite of the absence of food. (C) 1997 Academic Press.