Maintenance of the phenology of the winter moth (Lepidoptera: Geometridae)

Adult winter moths (Operophtera brumata (L.)) are active in late autumn or early winter. The eggs overwinter in the canopy of trees and hatch simultaneously with the bursting of host tree buds. Many young larvae disperse on the wind on silk strands. Larvae are polyphagus and feed until late spring when they pupate in soil or leaf litter. The duration of the egg and pupal stages is genetically determined and varies with latitude. The egg stage is long in the north and short in the south, while the pupal stage is short in the north and long in the south. The literature on the ecology and physiology of winter moth is reviewed. The factors maintaining the unusual phenology are discussed. It is concluded that the larval stage is early because mature leaves of many host trees are unsuitable as food, because parasitism against later larvae is more intense, and because summer temperatures may be injurious to larvae. The adult period is late in the year so that the final stages of pupal development occur in cool conditions and so that adults emerge after most insect predators have ceased activity. Throughout most of the range retarding the adult emergence period would cause activity to be impeded by severe winter weather; in the south this is not so and it is suggested that eggs must be on the trees for a minimum period to ensure synchronization of egg hatch with bud burst. The protracted adult emergence period may be an adaptation reducing predation by birds.