Why Are My Broccoli Stems Covered in Grey Aphids?

Dense, powdery grey colonies on broccoli, cauliflower or kale stems and the underside of leaves are mealy cabbage aphids (Brevicoryne brassicae). They are one of the most damaging brassica pests in the summer garden. Unlike the relatively benign blackfly on beans, mealy cabbage aphids feed in enormous colonies, cause significant stunting and leaf curl, and are protected by a waxy grey coating that makes them harder to dislodge than most aphid species.

Identifying mealy cabbage aphids

These aphids are grey-green with a distinctive mealy or powdery wax coating. They cluster in dense, tightly-packed masses on young stems, growing tips and leaf undersides. Infested tissue puckers, curls and discolours. In heavy infestations the plant's growing tip may be so congested that it stops developing entirely. Ants attending the colony are a sign of serious infestation — ants farm aphids for their honeydew secretions and actively defend them from predators.

Physical removal and water jets

Small colonies can be crushed by hand. Larger infestations respond to high-pressure water jets aimed directly at the colony, which physically dislodges the aphids. The waxy coating means a single spray is rarely sufficient — repeat every two to three days. Removing and binning heavily colonised shoot tips prevents the colony from re-establishing in the same place. Do not compost infested material as aphid eggs can survive.

Insecticidal soap and organic sprays

Insecticidal soap or a diluted solution of washing-up liquid and water (a few drops per litre) penetrates the waxy coating when applied thoroughly and kills aphids on contact. Pyrethrin or neem oil sprays are also effective. Apply in the evening to reduce impact on pollinators and repeat after rain. Avoid spraying when beneficial insects are abundant — populations of ladybirds, hoverfly larvae and parasitic wasps will suppress aphid numbers naturally if given the chance.

Prevention the following season

Inspect transplants before planting to ensure they are not already hosting aphid eggs on stems. Fine mesh netting prevents flying aphids from landing and establishing colonies. Encouraging a diversity of flowering plants near the brassica bed supports populations of aphid predators throughout the season.