Aphid (greenfly & blackfly)

What aphids actually are

Aphids are tiny, soft-bodied insects, usually only 1–3mm long, that feed by piercing plant tissue and sucking out the sap. "Greenfly" and "blackfly" are just the everyday names for the most familiar colours — there are hundreds of UK species, and they also come in grey, pink and yellow. Whatever the colour, the behaviour is the same.

They breed at an astonishing rate. Through the warmer months females give birth to live young without needing to mate, so a handful of aphids can become a dense colony within a week or two. This is why they seem to appear overnight on the soft new tips of broad beans, roses, courgettes and almost anything growing under glass.

The damage they do

Aphids weaken a plant in three ways. First, by draining sap they sap its vigour, distorting and curling young leaves and stunting fresh growth. Second, they excrete a sugary waste called honeydew that coats the leaves in a sticky film. A black, sooty-looking mould then grows on that honeydew — harmless in itself, but it blocks light from the leaf and makes everything look grubby.

Third, and most seriously for some crops, aphids spread plant viruses as they move from plant to plant. This matters far more on things like courgettes and other cucurbits than the feeding damage itself.

You will often spot ants patrolling an infested plant. They farm aphids for the honeydew and will even defend them, so a trail of ants up a stem is a useful early warning to go and look.

Organic control

The good news is aphids are easy to manage without reaching for chemicals, and a healthy garden usually keeps them in check by itself.

• Squash them. On a few plants, simply running finger and thumb up the affected shoots crushes the colony. It is the fastest fix for a small outbreak on broad beans or roses.
• Blast with water. A firm jet from the hose or a spray bottle knocks colonies off leaves and undersides; most can't climb back. Repeat every few days while numbers are high.
• Pinch out the tips. On broad beans, removing the soft growing tips once the lower pods have set removes the part blackfly love most. This doubles as pinching out and won't harm the crop.
• Encourage predators. Ladybirds, lacewings, hoverflies and small birds devour huge numbers of aphids. Growing flowers such as annuals like calendula and poached-egg plant draws these helpers in, so avoid spraying anything that would kill them too.

If you do want a treatment, a plain soft-soap or insecticidal soap spray sold by garden centres such as B&Q or Wilko is the gentlest option. For more detail, see our guide to greenfly and aphids.