Aphids cause damage by sucking the juices from the plant. Gathering in great numbers, aphids are most commonly found on the tips and stems of plants sucking the sap for nourishment. Aphids can also be found on the underside of foliage sucking, which leaves the leaf curled or puckered. The feeding by aphids results in a sticky liquid appearing on parts of the plants. This residue is often referred to as honeydew which can attract ants.
Ants can actually keep herds of aphids for a supply of the honeydew which is a food source.
The aphid excretion is often attacked by a secondary fungus which results in a blackening on the foliage called sooty mold. This damage is not harmful to the plant and in many cases is the first time many people notice aphids feeding on trees and shrubs. Minor aphid feeding is not damaging to the plants, but in some cases the feeding can be severe causing curling and wilting, and in some rare cases can carry several plant diseases. Depending upon how heavy the infestation is, you have several control options. Aphids can be knocked off with a stream of water, or picked off for those less squeamish.