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 consta nt temperature of 25 degreesC after transfer from short (12L:12D) to long d ay conditions (18L:6D). Higher diapause intensity was induced with a thermo period than at constant temperatures. After the induction throughout larval instars 3-5 and during 4 weeks of adult life at short days and a thermoper iod of 25/15 degreesC the pre-oviposition period was 30+/-4 and 26+/-3 days . After induction at constant 25 degreesC the pre-oviposition period was 22 +/-3 and 23+/-4 days, while after induction at constant 20 degreesC it was 17+/-4 and 19+/-4 days. Induction at a lower constant temperature of 20 deg reesC was thus followed by a less intense diapause than the induction at a higher constant temperature of 25 degreesC. These counterintuitive results are discussed. The oxygen consumption rate measured at experimental tempera tures prior to transfer from short to long days was higher at thermoperiodi c conditions than at constant temperatures and it was similar at constant 2 0 and 25 degreesC. Thus, the oxygen consumption rate measured prior to the transfer was highest (indication of the least intense diapause) in the inse cts that showed later, after the transfer to long days, the longest pre-ovi position period (indication of the most intense diapause). Within the first two days after transfer to constant 25 degreesC, oxygen consumption rate m easured at 25 degreesC decreased in the thermoperiodic insects: while it tr ansiently increased in insects from constant 20 degreesC. Two days and late r after the transfer, oxygen consumption rate was similar in all groups. Co ld hardiness was not correlated with diapause intensity. The low lethal tem perature in diapausing insects was correlated with the night temperature du ring diapause induction. (C) 2000 Elsevier Science Ltd. All rights reserved .