Thermal response and reversibility of prolonged larval diapause in the chestnut weevil, Curculio sikkimensis

Curculio sikkimensis undergoes prolonged larval diapause that is terminated by chilling and warming cycles. To examine the effects of warming temperatures and their duration on diapause termination, we exposed diapause larvae that had not been reactivated after chilling at 5 degrees C to 20 or 25 degrees C and chilled them again before incubation at 20 degrees C. With increasing warming duration at 20 degrees C, diapause termination after chilling increased and shorter chilling durations became effective. In contrast, few or no larvae warmed at 25 degrees C terminated diapause after chilling, irrespective of the warming duration. To investigate the effect of warming temperature on diapause intensity, larvae with diapause weakened by initial incubation at 20 degrees C after the first chilling were subsequently incubated at 15, 20, or 25 degrees C, then chilled at 5 degrees C before incubation at 20 degrees C. Diapause termination increased significantly after the larvae were treated at 15 or 20 degrees C but decreased significantly after they were treated at 25 degrees C. The intensification of prolonged diapause at 25 degrees C was reversed when the larvae were transferred to 20 degrees C. Diapause intensity in C. sikkimensis therefore decreases at 20 degrees C, increases at 25 degrees C, and can be reversed by alternately exposing diapause larvae to 20 and 25 degrees C. In C. sikkimensis, prolonged diapause does not always proceed in one direction, and its intensity fluctuates in response to ambient temperature conditions.