Aphids are a sucking insect that have a large number of natural enemies including lady beetles, lacewings, hoverflies and parasitic wasps. 

Nymphs and adults feed on plant juices, attacking leaves, stems, buds, flowers, fruit and or roots, depending on the species. Look for misshapen, stunted, curling or yellowing leaves.

If the stems or leaves are covered with a sticky substance, it may be the "honeydew" left from them sucking the sap. The honeydew can sometimes encourage fungal growth called sooty mould, causing branches and leaves to appear black.

-Avoid high nitrogen fertilisers as high nitrogen levels stimulate aphid reproduction.

- Use horticultural glues to keep ants that farm aphids out of your citrus trees. This will make it a lot easier for the predators to pick the aphids off.

- Spray infested plants with a strong stream of water to blast them off.

- Neem oil, insecticidal soaps and horticultural oils are effective if they come into direct contact with the aphids.