Egg development and diapause: ecophysiological and genetic basis of phenological polymorphism and adaptation to varied hosts in the green oak tortrix, Tortrix viridana L-(Lepidoptera : Tortricidae)

Experiments concerned 8 tortrix populations associated with varied oak spec ies. They showed that the egg development included a phase of diapause. Com pletion of embryogenesis at 20 degrees C was used as a criterion for whethe r diapause was completed. Under semi-natural conditions diapause terminated in late autumn or early winter, then eggs developed continuously, without postdiapause winter quiescence, even in severe cold. The eggs from the popu lations associated with helm or cork oak completed diapause then hatched la ter than those from the populations associated with sessile or pubescent oa k. Reciprocal crossbreedings confirmed that this phenological polymorphism was genetically determined. Under constant temperatures the physiological s tate of diapausing eggs, assessed by measuring their cold requirements to c omplete diapause by exposure to 8 degrees C, varied gradually with increasi ng age. This diapause development was strongly temperature-dependent. Cold requirements of diapausing eggs were much higher in a late-hatching than in an early-hatching population. This explains the phenological polymorphism of the tortrix: the more eggs need cold, the later they complete diapause i n autumn, and the later they hatch in spring. Egg cold requirements varied widely within populations too, which resulted in large variations in the da te of diapause end among individuals. The dare of egg hatch was influenced by the temperatures occurring during diapause and postdiapause, but apparen tly not by photoperiod. (C) 1999 Elsevier Science Ltd. All rights reserved.