Abstract
1. When spring migrants of Aphis fabae, Scop., from Euonymus europaeus and Viburnum opulus are given a choice of summer host plants enclosed by a muslin cage, they colonise bean first and then sugar beet. Dock, poppy and Chenopodium are colonised in no very definite order, but thistle is the last plant to be infested.2. Further infestation of surrounding plants takes place rapidly by means of apterae, which migrate actively and colonise plants at random.3. The total number of Aphids on the summer host plants is of the same order as the times of colonisation.4. The effect of parasites and predators is proportional to the numbers present and to the size of the colony.5. The results do not indicate the presence of biological races on the chosen host plants.I should like to thank Mr. F. G. W. Jones, of the School of Agriculture, Cambridge, for his help in preparing the ground for the plots and in rearing some of the plants, and Dr. W. H. Thorpe for his criticism.
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