Late autumn eclosion in the winter moth Operophtera brumata: compromise ofselective forces in life-cycle timing

1. In eclosion experiments at constant temperatures (6, 9, 12, 15, 18, 21, and 25 degrees C), Operophtera brumata (L.) pupae were found to respond non linearly to temperature, with 9 degrees C giving the highest developmental rate. 2. Pupal development rate decreased and mortality increased at the highest and lowest temperatures. No pupae eclosed at 6, 21, or 25 degrees C. 3. Exposing pupae to periods of cold did not enhance their rate of developm ent consistently, indicating that no pupal diapause occurred. Light did not affect the length of the pupal period significantly. 4. Variation in mean developmental rates across temperatures was modelled b oth for data in the present study (northern Norway) and for a previously pu blished German study (20 degrees further south). The German population had a longer pupal period at all temperatures than the northern population (mea n difference of 88 days). The difference is assumed to be genetically based , and hence an adaption to (and not a consequence of) differences in phenol ogy between the two sites. 5. The diversity of the life cycle of O. brumata is discussed. Timing of ec losion can be explained by the selective forces of predation by birds on th e one hand and mortality due to early snowfall on the other.