Healthy plants can tolerate a small number of aphids, however, damage results when large numbers are present. A short pair of tubes which are located on the abdomen are used for defensive purposes as they produce a spray warning when the aphid is threatened. Most aphids are wingless. However, when the temperature is not suitable or over-crowding takes place, some species can form wings and move to new areas. When a plant is attacked by a large population of aphids, damage such as reduced growth, wilted leaves, drying branches, stunted needles, curled, yellowed or downward cupped foliage may result.
Many aphids remove excess sap from plants and excrete it.
This clear, sticky, sweet substance, called honeydew is found below the host plant and may act as an attractant for other insect pests such as ants. The excess honeydew also acts as a host for a harmless black sooty mold which often grows in the secretion. Some species of aphids are also known to transmit viruses to the host plant upon which it feeds. Aphid populations can reach high numbers very quickly. This is possible because they have a high reproductive rate, some females can give birth to nymphs without mating and a large percentage of the offspring are females.