About 4,400 species of 10 families are known. Historically, many fewer families were recognized, as most species were included in the family Aphididae. Around 250 species are serious pests for agriculture and forestry as well as an annoyance for gardeners. They vary in length from one to ten millimetres.
Natural enemies include predatory lady beetles (Coleoptera: Coccinellidae), hoverfly larvae (Diptera: Syrphidae), parasitic wasps, aphid midge larvae, aphid lions, crab spiders lacewings (Neuroptera: Chrysopidae), and entomopathogenic fungi like Lecanicillium lecanii and the Entomophthorales.
Aphids are distributed worldwide, but are most common in temperate zones. Also, in contrast to many taxa, species diversity is much lower in the tropics than in the temperate zones. They can migrate great distances, mainly through passive dispersal by riding on winds. For example, the currant lettuce aphid (Nasonovia ribisnigri Mosley) is believed to have spread from New Zealand to Tasmania in this way. Aphids have also been spread by human transportation of infested plant materials.
Like aphids, phylloxera feed on the roots, leaves and shoots of grape plants, but unlike aphids do not produce honeydew or cornicle secretions. Phylloxera (Daktulosphaira vitifoliae), are insects which caused the Great French Wine Blight and a plague that devastated European viticulture in the 19th century.
Most aphids have soft, green bodies, but other colors are common, such as black, brown, and pink. Aphids have antennas with as many as six segments. Aphids feed themselves through sucking mouthparts called stylets, enclosed in a sheath called a rostrum, which is formed from modifications of the mandible and maxilla. They have long, thin legs and two-jointed, two-clawed tarsi.
Most aphids have a pair of cornicles (or "siphunculi"), abdominal tubes through which they exude droplets of a quick-hardening defensive fluid containing triacylglycerols, called cornicle wax. Other defensive compounds can also be produced by some types of aphids.
Aphids have a tail-like protrustion called a "cauda" above their rectal apertures. They have two compound eyes, and an ocular tubercle behind and above each eye, made up of three lenses (called triommatidia).
When host plant quality becomes poor or conditions become crowded, some aphid species produce winged offspring, "alates", that can disperse to other food sources. The mouthparts or eyes are smaller or missing in some species and forms.
Many aphid species are monophagous (that is, they feed on only one plant species). Others, like the green peach aphid (Myzus persicae), feed on hundreds of plant species across many families.
Aphids passively feed on sap of phloem vessels in plants, as do many of their fellow members of Hemiptera such as scale insects and cicadas. Once a phloem vessel is punctured, the sap, which is under high pressure, is forced into the aphid's food canal. As they feed, aphids often transmit plant viruses to the plants, such as to potatoes, cereals, sugarbeets and citrus plants. These viruses can sometimes kill the plants.
Plants contain low densities of the nitrogen compounds needed for building proteins. This requires aphids to consume an excess of sap to satisfy their nutritional requirements. The excess is expelled as "honeydew", out of the recta of aphids, in such large volumes that in sometimes it can "fall like rain". Aphid honeydew is rich in carbohydrates, like the phloem it derives from.
Some species of ants "farm" aphids, protecting them on the plants they eat, and eating the honeydew that the aphids release from the terminations of their alimentary canals. This is a "mutualistic relationship".
These "dairying ants" "milk" the aphids by stroking them with their antennae. Therefore, sometimes aphids are called "ant cows".
Some farming ant species gather and store the aphid eggs in their nests over the winter. In the spring, the ants carry the newly-hatched aphids back to the plants. Some species of dairying ants (such as the European yellow meadow ant, Lasius flavus) manage large "herds" of aphids that feed on roots of plants in the ant colony. Queens that are leaving to start a new colony take an aphid egg to found a new herd of underground aphids in the new colony. These farming ants protect the aphids by fighting off aphid predators.
An interesting variation in ant-aphid relationships involves Lycaenid butterflies (such as the Sievers blue butterfly and the Japanese copper butterfly) and the Myrmica ants. For example, Niphanda fusca butterflies lay eggs on plants where ants tend herds of aphids. The eggs hatch as caterpillars which feed on the aphids. The ants do not defend the aphids from the caterpillars, but carry the caterpillars to their nest. In the nest, the ants feed the caterpillars, which produce honeydew for the ants. When the caterpillars reach full size, they crawl to the colony entrance and form cocoons. After two weeks, butterflies emerge and take flight.
Many aphids are host to endosymbiont bacteria, Buchnera, inside specialized cells called bacteriocytes. These bacteria synthesize some essential amino acids that are absent from the phloem that the aphids eat.
Some aphid species have unusual and complex reproductive adaptations, while others have fairly simple reproduction. Adaptations include having both sexual and asexual reproduction, creation of eggs or live nymphs and switches between woody and herbaceous types of host plant at different times of the year.
Many aphids undergo cyclical parthenogenesis. In the spring and summer, mostly or only females are present in the population. The overwintering eggs that hatch in the spring result in females, called fundatrices. Reproduction is typically parthenogenetic and viviparous. Females undergo a modified meiosis that results in eggs that are genetically identical to their mother (parthenogenetic). The embryos develop within the mothers' ovarioles, which then give live birth to first instar female nymphs (viviparous). The offspring resemble their parent in every way except size, and are called virginoparae.
This process iterates throughout the summer, producing multiple generations that typically live 20 to 40 days. Thus one female hatched in spring may produce many billions of descendants. For example, some species of cabbage aphids (like Brevicoryne brassicae) can produce up to 41 generations of females, or more than 1.5 x 1027 offspring if they all live.
In autumn, aphids undergo sexual, oviparous reproduction. A change in photoperiod and temperature, or perhaps a lower food quantity or quality, causes females to parthenogenetically produce sexual females and males. The males are genetically identical to their mothers except that they have one less sex chromosome. These sexual aphids may lack wings or even mouthparts. Sexual females and males mate, and females lay eggs that develop outside the mother. The eggs endure the winter and emerge as winged or wingless females the following spring. This is, for example, the life cycle of the rose aphid (Aphis rosae), which may be considered typical of the family. However in warm environments, such as a in the tropics or in a glasshouse or greenhouse, aphids may go on reproducing asexually for many years.
Some species produce winged females in the summer, sometimes in response to low food quality or quantity. The winged females migrate to start new colonies on a new plant, often of quite a different kind. For example, the apple aphid (Aphis mali), after producing many generations of apterous females on its typical food-plant, gives rise to winged forms which fly away and settle on grass or corn-stalks.
Some aphids have telescoping generations. That is, the parthenogenetic, viviparous female has a daughter within her, who is already parthenogenetically producing her own daughter. Thus a female's diet can affect the body size and birth rate of more than one generation (daughters and granddaughters).
Insects that attack aphids include predatory lady beetles (Coleoptera: Coccinellidae), hoverfly larvae (Diptera: Syrphidae), parasitic wasps, aphid midge larvae, aphid lions, crab spiders and lacewings (Neuroptera: Chrysopidae).
Fungi that attack aphids include Neozygites fresenii, Entomophthora, Beauveria bassiana, Metarhizium anisopliae and entomopathogenic fungi like Lecanicillium lecanii. Aphids brush against the spores, which are typically about 1/4 inch long. These spores stick to the aphid, germinate and penetrate the aphid's skin. The fungus grows in the aphid hemolymph (i.e., the counterpart of blood for aphids). After about 3 days, the aphid dies and the fungus releases more spores into the air. Infected aphids are covered with a woolly mass that progressively grows thicker until the aphid is obscured. Often the visible fungus is not the type of fungus that killed the aphid, but a secondary fungus.
Aphids can be easily killed by unfavorable weather, such as late spring freezes. Excessive heat kills the symbiotic bacteria that some aphids depend on, which makes the aphids infertile. Rain prevents winged aphids from dispersing, and knocks aphids off plants and thus kills them from the impact or by starvation. However, Ken Ostlie, an entomologist with the University of Minnesota Extension Service, suggests that rain should not be relied on for aphid control.
Aphids are soft-bodied, and have little protection from predators and diseases. Some species of aphid interact with plant tissues forming a gall, an abnormal swelling of plant tissue. Aphids can live inside the gall, which provides protection from predators and the elements. A number of galling aphid species are known to produce specialised "soldier" forms, sterile nymphs with defensive features which defend the gall from invasion. For example, Alexander's horned aphids are a type of soldier aphid that has a hard exoskeleton and pincer-like mouthparts. Infestation of a variety of Chinese trees by Chinese sumac aphids (Melaphis chinensis Bell) can create a "Chinese gall" which is valued as a commercial product. As "Galla Chinensis", Chinese galls are used as a medication used in Chinese medicine to treat coughs, diarrhea, night sweats, dysentry and to stop intestinal and uterine bleeding. Chinese galls are also an important source of tannins.
Some species of aphid, known as "woolly aphids" (Eriosomatinae), excrete a "fluffy wax coating" for protection.
It was common at one time to suggest that the cornicles were the source of the honeydew, and this was even included in the Shorter Oxford English Dictionary and the 2008 edition of the World Book Encyclopedia.. In fact, honeydew secretions are produced from the anus of the aphid, while cornicles mostly produce defensive chemicals such as waxes. There also is evidence of cornicle wax attracting aphid predators in some cases. Aphids are also known to defend themselves from attack by parasitoid wasps by kicking.
The coating of plants with honeydew can contribute to the spread of fungi which can damage plants. Honeydew produced by aphids has been observed to reduce the effectiveness of fungicides as well.
A hypothesis that insect feeding may improve plant fitness was floated in the mid-1970s by Owen and Wiegert. It was felt that the excess honeydew would nourish soil micro-organisms, including nitrogen fixers. In a nitrogen poor environment, this could provide an advantage to an infested plant over a noninfested plant. However, this does not appear to be supported by the observational evidence.
The damage of plants, and in particular commercial crops, has resulted in large amounts of resources and efforts being spent attempting to control the activities of aphids.
In the movie Antz, Weaver says to Zee "Don't you want your aphid beer?" Zee replies "I can't help it. I have a thing about drinking from the anus of another creature. Call me crazy."