The life‐cycle and host preferences of the bird cherry‐oat aphid, Rhopalosiphum padi L., and their bearing on the theories of host alternation in aphids

SUMMARY Emigrants of the host‐alternating bird cherry‐oat aphid fly from the primary host to various grasses before the end of June, even in the absence of natural enemies on bird cherry. They fly before the leaves of bird cherry mature and at a time when numbers of insect predators are rapidly increasing. In the autumn gynoparae and males return to bird cherry whose senescing leaves again provide the aphids with a rich source of food. The growth efficiency of R. padi on bird cherry and oats indicates that the quality of the available food on the two hosts is markedly different. Apterous exules prefer oats to young bird cherry leaves and survive best on oats. Gynoparae prefer bird cherry leaves and only successfully produce offspring on the primary host. This, and other work published on R. padi , support the premise that a change in the aphid's preferences causes the change of host. The dual‐discrimination theory of host alternation in aphids is examined in the light of this and other published work.