The role of cereal growth stages and crowding in the induction of alatae in Sitobion avenae and its consequences for population growth

Abstract. 1. Alatiform nymphs of Sitobion avenae were first recorded on wheat in 1977 and 1978 when there were 0.6 aphids per tiller and reached a maximum proportion of 0.9 of the population despite a seventeen-fold difference in aphid density between years.
2. At the same aphid density per tiller there was a higher proportion of alatiform nymphs on the tillers at an advanced than at an earlier stage of plant growth.
3. Both pre- and post-natal crowding, and late stages of plant growth, were important in inducing the development of alatae in the laboratory.
4. Changes in the quality of the host plant through its previously reported effect on reproductive and survival rates, and its effect on alate induction and emigration was important in shaping the population dynamics of S.avenae on cereals.