Data on the populations of the Pea Aphid, Acyrthosiphon pisum (Harris), have been accumulated over four years on several leguminous crops in East Anglia. The populations on lucerne have provided the most extensive studies.

Considerable annual variation was shown by the populations, but in spring the development of a peak in the numbers of Aphids occurred on lucerne, hop trefoil and sainfoin. On lucerne, which was periodically cut, the Aphids persisted throughout the growing season and a tendency towards the development of a peak in numbers in the autumn was also recorded.

All the factors which affected the number of Aphids varied considerably in the influence they exerted. The probable reason for the more obvious trends in the populations have been given in fairly broad outline, but how the effect of these factors had been altered or predisposed by less apparent factors has been speculated upon only in certain instances.