Autumnal moth - why autumnal?

1. As for some other spring-feeding moths, adult flight of Epirrita autumna ta (Lepidoptera: Geometridae) occurs in late autumn. Late-season flight is a result of a prolonged pupal period. Potential evolutionary explanations f or this phenological pattern are evaluated. 2. In a laboratory rearing, there was a weak correlation between pupation d ate and the time of adult emergence. A substantial genetic difference in pu pal period was found between two geographic populations. Adaptive evolution of eclosion time can thus be expected. 3. Metabolic costs of a prolonged pupal period were found to be moderate bu t still of some ecological significance. Pupal mortality is likely to form the main cost of the prolonged pupal period. 4. Mortality rates of adults. exposed in the field, showed a declining temp oral trend from late summer to normal eclosion time in autumn. Lower predat ion pressure on adults may constitute the decisive selective advantage of l ate-season flight. It is suggested that ants, not birds, were the main pred ators responsible for the temporal trend. 5. Egg mortality was estimated to be low; it is thus unlikely that the late adult period is selected for to reduce the time during which eggs are expo sed to predators. 6. In a laboratory experiment, oviposition success was maximal at the time of actual flight peak of E. autumnata, however penalties resulting from sub -optimal timing of of oviposition remained limited.