Aphids feed on new vegetative growth and flowering sites. They suck up the sugary liquid from the phloem tissues using a straw-like proboscis (elongated nose or snout).
It’s startling how quickly they can lay eggs and grow. And they munch up masses of plant material, maiming your leaves and stems. If left untreated, eventually your plants will die.
Watch out for leaf and stem deformities. You might also spot drops of honey dew on leaves – a waste product that aphids shed onto lower leaves. Eventually, this sticky substance changes into a black sooty mould.
Cut off the most infected parts of the plant, including many of the new flowering sites. Then use a decent insecticide spray, such as Pyrethrum 5 EC / Protector.