Why Are There Masses of Blackfly on My Bean Shoot Tips?

Dense, heaving colonies of black aphids clustering on bean growing tips, tender stems, and the undersides of young leaves are caused by the black bean aphid, Aphis fabae — one of the most familiar pests of UK gardens. On runner beans and French beans it follows an almost predictable seasonal pattern: winged adults arrive from overwintering woody hosts (particularly spindle, Euonymus europaeus) in late spring to early summer, colonise the most tender growth at the shoot tips, and within days a small founding colony becomes hundreds or thousands of individuals reproducing without mating.

The damage they cause

Heavy aphid infestations on growing tips distort and weaken the new growth, reduce vigour, contaminate developing pods with sticky honeydew (which grows black sooty mould), and — most significantly — transmit viral diseases including bean mosaic virus. Aphids feeding on a plant for even a few seconds can transmit virus before a gardener notices them. This is why early intervention, before populations explode, is far more effective than trying to manage a very large infestation later.

Physical removal — the most effective method

Crush aphid colonies on shoot tips by running the shoot between finger and thumb — this is fast, cost-free, and highly effective on small to medium infestations. Alternatively, cut off and bin the infested shoot tips, removing the colony entirely. A firm jet of water from a hose dislodges aphids from stems and leaves — many do not return. Ladybird larvae, lacewing larvae, and hoverfly larvae consume large numbers of aphids; avoiding pesticides that kill these predators keeps natural control working throughout the season.

The pinching-out strategy

Once runner beans reach the top of their support poles (typically 1.8–2 m), pinch out the growing tip entirely. This is standard growing practice that concentrates the plant's energy into pod production rather than further vegetative growth — and it simultaneously removes the most aphid-attractive tissue. On French beans, which are self-stopping, pinching is less important but can be done on any tip showing a large aphid colony.