Role of Photoperiod in the Timing of Dispersal in the Green Spruce Aphid Elatobium abietinum

Abstract

(1) The time of development of alatae (migrants) of the green spruce aphid, Elatobium abietinum (Walker), is described, and the factors that induce the development of migrants are investigated. (2) Migrants develop only in spring and early summer; the proportion depends on date of birth. (3) At a constant aphid density under natural conditions of increasing day-length the pattern of migrant production was similar to that recorded by suction traps. (4) Although crowding increased the proportion of migrants they also developed in isolation. Pre-natal crowding was more important than post-natal crowding, but the greatest proportion of migrants developed amongst aphids crowded both preand post-natally. (5) Proportionately more migrants developed on host-plants in the early stages of bud expansion and stem elongation than on dormant hosts. (6) A photoperiod increase was the major factor inducing the development of migrants, and host quality and crowding modified the response. Day-length probably acts directly on the aphid rather than indirectly through the plant. (7) The timing of the development of migrants and their dispersal and the proportion of migrants that develop is discussed in relation to the seasonal trend and aphid-induced changes in habitat quality, and the variability in quality between host-plants.