Black Bean Aphid (Blackfly): how to recognise and treat it

Pest

Symptoms

Dense colonies of black aphids up to 2 mm long, sometimes with white flecks, on soft shoot tips, flower stems and the undersides of young leaves.
Ants running over the colonies, sticky honeydew on the leaves, often with black sooty mould, and whitish cast skins.
Distorted shoots and stunted plants; broad beans set fewer pods, and flowers of dahlias and nasturtiums can be damaged.
In spring, curled leaves on its winter hosts — viburnum, mock orange and spindle; this damage is minor and the shrubs recover.

Squash colonies between finger and thumb and pinch out heavily infested shoot tips.
On broad beans, pinch out the shoot tips once the lowest flowers have formed small pods; blackfly colonise the soft tips, and the tips can be cooked and eaten.
Knock aphids off sturdy plants with a strong jet of water, or spray insecticidal soap or a plant oil such as neem, wetting the leaf undersides; it kills only the aphids it hits, so repeat. Do not spray drought-stressed plants or in hot weather.

The RHS advises gardeners not to use pesticides against blackfly; if a spray is unavoidable, choose insecticidal soap combined with pyrethrins, which break down quickly — spray in the evening away from open flowers and follow the label dose and pre-harvest interval.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Check bean tips frequently from spring so you can act before colonies build up.
Encourage ladybirds, hoverflies, ground beetles, parasitoid wasps and earwigs; in spring, aphid numbers rise before their enemies catch up, so wait a little before acting.
Do not overfeed with nitrogen — use slow-release fertiliser; soft lush growth favours aphids.

Figures checked against these sources.