Aphids (greenfly & blackfly)
How to recognise it
Clusters of small green, black or grey insects on soft shoot tips, buds and the undersides of young leaves, often with a sticky honeydew residue and sooty mould developing on it.
Life cycle and conditions
Breed rapidly in mild, humid spring and summer weather; many species overwinter as eggs on woody stems, hatching as temperatures rise in spring.
Prevention
Encourage natural predators (ladybirds, hoverfly larvae, birds) by avoiding blanket spraying; check vulnerable soft growth regularly from spring onward.
How to treat it
Squash small colonies by hand or dislodge with a strong jet of water; a soap or fatty-acid based spray deals with heavier infestations without harming visiting bees once dry. Encouraging ladybirds and hoverflies gives lasting, season-on-season control.
Only if the bio option fails and the infestation threatens the plant: use an insecticide approved for amateur UK garden use, checking current approval status first. Broad-spectrum products also kill the aphid predators that would otherwise keep numbers down for you.