Aphids
Small, soft-bodied insects that cluster on new shoots and buds, sucking sap and leaving sticky honeydew behind.
Also known as: Greenfly, Blackfly, Plant lice
Symptoms and identification
Clusters of tiny green, black, grey or yellow insects on shoot tips, buds and leaf undersides. New leaves curl or look twisted, buds may fail to open, and leaves become sticky with honeydew that later turns black with sooty mould. Ants running up and down the stems are often a clue.
Causes
Aphids multiply very quickly in warm, dry spells, and females can produce young without mating. Soft, lush growth from too much nitrogen fertilizer attracts them, and ants protect them from predators in exchange for honeydew.
Organic treatment
Knock aphids off with a firm spray of water. For larger colonies, spray insecticidal soap or neem oil (about 5 ml of cold-pressed neem oil and 2 ml of liquid soap per litre of water), covering the undersides of leaves. Spray in the evening and repeat every 5–7 days until they are gone. Prune off badly infested tips.
Prevention
Inspect new growth once a week. Avoid heavy nitrogen feeding, keep plants well watered, and encourage ladybirds, lacewings and hoverflies by growing flowers such as marigold and coriander nearby. Control ants on the plant.
Chemical treatment (use with care)
If an infestation keeps coming back, use a contact insecticide labelled for aphids, such as one based on pyrethrins. Follow the label, do not spray open flowers visited by bees, and observe the waiting period before harvesting edible herbs.