Mortality during dispersal and the cost of host-specificity in parasites: how many aphids find hosts?

1. For a full assessment of explanations for the evolution of host-specificity it is necessary to estimate the probability that a dispersing parasite finds a host. We develop a method of estimating this success rate from samples of dispersing parasites and populations resident on hosts. 2. Applying this method to data on the bird cherry-oat aphid, Rhopalosiphum padi (L.), from southern Scotland in 1984-92, we estimate that 0.6% of the autumn migrants find hosts. 3. With such a low success rate, there should be selection for a broadening of host range, to include any host on which the colonist's fitness is more than about 0.6% of that on the normal hosts. We argue that neither nutrition nor the need for 'enemy-free space' are sufficient explanations of the host-specificity of this animal, and propose instead that it is the host's role as a rendezvous for mating that constrains the migrants to their costly host-specificity. 4. We also discuss the implications of this low success rate for the hypothesis that aphids speciate sympatrically through the formation of host races.