Aphids belong to order Homoptera, along with leafhoppers, whiteflies and scales. They undergo incomplete metamorphosis (hemimetabolic insects). Their life cycle includes three main stages: egg, larva and adult.
Eggs: Small, black and shiny.
Larvae: Resemble adults, only smaller.
Adults: Soft, pear-shaped body, 1 to 4 mm long. They range in colour from green to red, black, pink, yellow, brown or bluish. Woolly aphids are covered in a waxy, white, cotton-batting- or wool-like substance. The head has two long antennae and an articulated rostrum (mouthparts) for sucking. The thorax has six slender legs. Most adult aphids are apterous (wingless). Winged aphids have two pairs of transparent, membranous wings. The abdomen has a sort of tail (cauda), through which the honeydew flows, and sometimes two tubes (cornicles), used for defence.
Aphids have a complex and unusual life cycle. Some aphids spend their entire life on the same plant, while others must change host plants to complete their life cycle.
Some species reproduce sexually, while others multiply solely by parthenogenesis (without fertilization by a male).
In temperate regions, aphids usually overwinter as eggs, hidden between the scales on buds, in cracks in bark or underground. Some ants that raise aphids for their honeydew shelter the eggs in their nests in winter. In spring, they transport them to host plants.
When the overwintered eggs hatch in spring, the first generation consists only of apterous (wingless) females. Once they mature (in approximately 8 days at 20°C), they do not need to be fertilized and, rather than laying eggs, give birth, by parthenogenesis, to tiny wingless female aphids. During their short lives (20 to 30 days), each female can produce from 40 to 100 aphids, which continue to reproduce in the same way.
Over the summer, winged females appear periodically and move to other plants to form new colonies.
In fall, the females give birth to winged males and wingless females. After mating, the fertilized females lay one to four eggs, attaching them to the stems of host plants or gaps in bark and buds. These eggs hatch the following spring.
There are several generations each year.