Effect of thermoperiod on diapause intensity in Pyrrhocoris apterus (Heteroptera Pyrrhocoridae)

The intensity of adult diapause in Pyrrhocoris apterus was measured in two series of experiments as the duration of pre-oviposition period at a constant temperature of 25°C after transfer from short (12L:12D) to long day conditions (18L:6D). Higher diapause intensity was induced with a thermoperiod than at constant temperatures. After the induction throughout larval instars 3–5 and during 4 weeks of adult life at short days and a thermoperiod of 25/15°C the pre-oviposition period was 30±4 and 26±3 days. After induction at constant 25°C the pre-oviposition period was 22±3 and 23±4 days, while after induction at constant 20°C it was 17±4 and 19±4 days. Induction at a lower constant temperature of 20°C was thus followed by a less intense diapause than the induction at a higher constant temperature of 25°C. These counterintuitive results are discussed. The oxygen consumption rate measured at experimental temperatures prior to transfer from short to long days was higher at thermoperiodic conditions than at constant temperatures and it was similar at constant 20 and 25°C. Thus, the oxygen consumption rate measured prior to the transfer was highest (indication of the least intense diapause) in the insects that showed later, after the transfer to long days, the longest pre-oviposition period (indication of the most intense diapause). Within the first two days after transfer to constant 25°C, oxygen consumption rate measured at 25°C decreased in the thermoperiodic insects, while it transiently increased in insects from constant 20°C. Two days and later after the transfer, oxygen consumption rate was similar in all groups. Cold hardiness was not correlated with diapause intensity. The low lethal temperature in diapausing insects was correlated with the night temperature during diapause induction.