There are about 4000 species of aphids worldwide, of which about 250 are serious pests. Aphids are garden pests as they feed on plant sap by inserting their stylets (mouthparts) into the plant's phloem. As well as damaging the plant, they can transmit plant viruses. Plant sap is high in sugars, but low in proteins, so aphids have to ingest large volumes of sap to get a balanced diet. This is why aphids produce honeydew, a sugary excretion, which can provide a substrate for sooty moulds. These moulds can grow anywhere where there is a high concentration of honeydew, such as in cars parked under lime trees in the late summer. If you must control aphids, remember all the beneficial insects that rely on them. Remove them manually or apply detergent-based sprays. If you must use pesticides, apply systemic ones such as primicarb, which should have a minimal effect on beneficial insects.
Aphids can reproduce parthenogenetically (asexually) for much of the time, with each female producing a series of genetically identical offspring.
Aphids are like Russian dolls. Inside every female is a developing nymph, which in turn contains a developing aphid embryo. Each female therefore carries her own granddaughters.
Most aphids have symbiotic microorganisms, called Buchnera, which help produce essential amino acids the aphid can not live without. Many aphid species are tended by ants. In return for protecting the aphids from predators, the ants are rewarded with honeydew.