Population Dynamics of the Cabbage Aphid, Brevicoryne brassicae (L.)

Abstract

The results of a 3-year field and laboratory study at Canberra, Australia, where the annual cycle of the aphid is simplified by the virtual suppression of the sexual phases, parthenogenetic reproduction continuing throughout the year. The new methods of instar-distribution analysis of field population samples are used to produce an integrated account of the actions and interactions of changes in reproductive rate, emigration, parasitism, predation, disease incidence, starvation and catastrophe. Aerial migration by winged forms is clearly essential to the survival of the species. After the initial colonization of a host-plant the aphid population increases rapidly until physiological changes occurring either together or independently within plant and aphid ensure that a large proportion of the progeny emigrate, escaping the direct consequences of local overpopulation. The survival of the host-plant species is ensured by plant regeneration after aphid damage and partly by interactions between emigration of the aphids, a fall in their reproductive rate, and an increase in their extrinsic mortalities, which together result in a rapid decline of aphid numbers from peak levels.