This section is from the book "Commercial Gardening Vol3", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
The curling of Plum leaves by Aphis is often very severe, and causes the leaves to fall prematurely. The green Aphides which cause this disease are found protected in the curled leaves, where they produce living young with great rapidity. They are the progeny of a dull plum - coloured mother queen Aphis which hatches out before the buds burst, and which may be found, long before there is any sign of leaf or blossom, seated in the axils of the buds. This fat female produces green living young which curl up the leaves. In June this Aphis becomes winged and leaves the Plum; where it goes to we do not know. It returns to the Plums in autumn and produces a sexual brood, the oviparous females laying eggs in and around the bud cluster, where they remain until the next February or March.
Treatment consists of early spraying with soft soap and quassia before the leaves become badly curled. Winter spraying in February with thick lime-and-salt wash will stop many of the ova from hatching out, and would destroy any of the plum-coloured mother queens.