Pest
Aphids
Bladlus
How to recognise it
Dense clusters of small green or black insects on stems and leaf undersides. Look for sticky honeydew coating the leaf surface and ants trailing up the stem (ants farm aphids for their honeydew).
Prevention
Companion-plant with garlic or chives, which emit sulphur compounds aphids avoid. Encourage natural predators such as ladybirds and hoverflies by planting sweet alyssum or marigolds nearby.
Treatment
Blast colonies off with a strong jet of water first; this alone is often enough. For persistent infestations, apply insecticidal soap or diluted neem oil, coating the undersides of leaves thoroughly. Repeat every 5–7 days.