Aphids cause considerable damage to the host plants. Being sap-sucking insects, they deplete the plants of nutrients and can cause wilting. Aphids also transmit viral diseases and the vast quantities of honeydew also attract fungal growth and ants.

Aphids are sucking insects with soft, rounded bodies. They range from 1/12" to 1/8" long and their color may vary. They occur in clusters on buds or new foliage. Aphids suck sap, causing reduced growth or stunting of the plant. Feeding by some species may produce distorted leaves. Aphids secrete "honeydew" which gives the leaves a shiny appearance. A black sooty mold may grow on the honeydew. 

Unthrifty or stunted plants and plants with curled or deformed leaves are likely to have aphid infestation. Light infestations are not harmful to plants, but higher populations cause leaf curl, wilting, stunting of shoot growth and delay in production of flowers and fruits as well as a general decline in plant vigour

Control: 

The key to prevent serious aphid damage is to check your garden regularly atleast twice a week. You examine your garden carefully for the signs of aphids. You have to look for clusters of the little bugs on new growth, flower buds and curled and twisted leaves.

If you see aphids remove them by hand wearing gloves. Spray a strong jet of water on infested parts. You should cut away or pinch off the infested leaves and bag them to dispose. A non-toxic pesticide you can make by mixing any vegetable oil and liquid soap. Spray this mixture thoroughly on infested plants. 

Avoid excess fertilisation, which stimulates tender and succulent growth. Fertilise sparingly using slow-releasing organic fertiliser to ensure steady growth, but not spurt growth, because too much new growth will attract more aphids. You should also avoid fertiliser containing highly soluble nitrogen, because aphids thrive on plants that have high levels of nitrogen in their leaves and buds.

You can use insecticides like Malathion only to the affected areas. As an alternative traditional chemical treatment try spraying with Neem (Azadirachtin), which is a non-toxic, biodegradable substance, which has natural insecticidal properties. Neem discourages aphids by making the plants unpalatable.