Population studies on the active stages of the black bean aphid, Aphis fabae Scop., on its winter host Euonymus europaeus L

SUMMARY Autumn populations of Aphis fabae Scop, on the primary host Euonymus europaeus L. were little affected by natural enemies, most of which had begun to hibernate before the aphid populations developed. The size of the population in spring was usually determined by the number of overwintering eggs on a bush. The fundatrices hatched about 3?6 weeks before natural enemies became common. The growth of large A. fabae populations was first halted by the effects of intra-specinc competition, notably by the production and departure of emigrant alatae and by adult apterae reproducing more slowly. Later, natural enemies, especially Adalia 2-punctata (L.), Syrphidae and the parasite Trioxys sp. (near angelicae), multiplied and accelerated the decline in the aphid populations, which usually disappeared in June leaving many immature natural enemies. Larval A. 2-punctata began to eat parasitized aphids and cannibalized other larvae and pupae. The small populations of A. fabae that develop from few overwintering eggs are at greater risk from natural enemies than are large ones. Intraspecific competition still slowed population increase, because most aphids remained crowded on the few originally colonized twigs. Such populations produced very few emigrant alatae before they were exterminated by the combination of T. angelicae with specific and non-specific predators. Adult Cantharidae killed many of the aphids, especially in hedgerow habitats, where they were abundant. Coccinellidae, Anthocoridae and syrphid larvae, and the adults of nonspecific predators, notably Cantharidae, prevented recolonization of E. europaeus throughout July and August. Leaves of E. europaeus may remain physiologically suitable for A. fabae throughout July but begin to deteriorate in August when A. fabae kept on them become less fecund. Experiments using exclusion techniques provided evidence that natural enemies which attack A. fabae on E. europaeus and on summer hosts cause the common 2-year cycle of aphid abundance. Individual E. europaeus differ consistently in the extent to which they are colonized by A. fabae. Conditions are discussed that should govern the choice of E. europaeus bushes on which the A. fabae populations can be used as sensitive indicators of later crop infestations.