Aphid (Aphididae's family), also known as plant louse, ant cow, or greenfly, any of a group of sap-sucking, soft-bodied insects (with the Homoptera order), which are around the size of a pinhead, most species of which hold a pair of tubelike projections (also called cornicles) on the abdomen. Aphids may be serious plant pests and can stunt plant growth, produce plant galls, transmit plant virus diseases, and cause the deformation of buds, leaves, and flowers.
Life Cycle of Aphids
Aphids' life cycle is complicated. Wingless females, known as stem mothers, reproduce without fertilization (it means by parthenogenesis) throughout the summer. These stem mothers are unique in that they produce living young (called viviparity) as opposed to the eggs, as takes place in most other insects. Eventually, the plant holding the stem mother and her offspring becomes overcrowded. When this takes place, a few offspring develop into adults having two pairs of large membranous wings.
These specific winged adults fly to new plants. In late summer, both the males and females are produced. After they both mate, the female lays eggs, which survive the winter. In warm climates, there can be no need for an overwintering egg stage, and continuous generations take place. The white woolly-ball appearance of several aphids is the result of wax-gland secretion. Often, aphids are controlled by natural enemies such as aphid lions, ladybird beetles, and lacewings. However, when aesthetically or economically damaging numbers are present, they can be controlled with the help of horticultural oils, insecticidal soaps, and other traditional aphid insecticides.
Oleander aphids (Aphis nerii) with eggs (centre right).
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Ants may guard and care for aphids in the honeydew return (a sweet excretory product) they produce. Ants protect aphids from natural and weather enemies and transfer them from wilted to healthy plants (aphids on plants). In this manner, the ants ensure their source of honeydew that they use as food. Ants obtain honeydew by stroking, or "milking," the aphids. Some common types of aphids are discussed here.
Types of Aphids
More than 4,000 species of aphids have been defined, a few 250 of which are pests of ornamental plants and crops. The life ecology, history and agricultural significance of selected species are defined here.
The apple aphid (which is Aphis pomi) is yellow-green with legs and a dark head. It overwinters as a black egg on its only host, which is the apple tree. It produces honeydew, which supports the growth of a sooty mold.
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The cabbage aphid (called Brevicoryne brassicae) is grey-green and small with a powdery, waxy covering. It is found in the clusters on the underside of leaves of cauliflower, cabbage, radishes and Brussels sprouts. It overwinters as black eggs in the northern regions but has zero sexual stage in southern regions. When needed, it can be controlled using aphid insecticides.
The Cooley spruce gall adelgid (which is Adelges cooley) causes the formation of the cone-like galls up to 7 cm (3 in) long on the tips of the spruce twigs. In midsummer times, when the galls open, adults will migrate to Douglas first to lay eggs. However, the life cycle can proceed on either Douglas fir or spruce. Control is by spraying with the insecticides by removing galls before aphids emerge and planting Douglas fir and spruce apart from each other.
The corn root aphid (or Anuraphis maidi radicis) is a serious pest, which is dependent on the cornfield ant. In winter, the ants store the aphid eggs in their nests and, in the spring season, carry the newly hatched aphids to weed roots, transferring them to corn roots whenever it is possible. The aphid stunts the growth of corn and it causes plants to turn wilt and yellow, which is called aphids on plants. Also, corn root aphids infest the other grasses.
The eastern spruce gall adelgid (which is Adelges abietis) produces the pineapple-shaped galls 1 to 2.5 cm (0.4 - 1 in) long composed of several cells, each containing up to 12 aphid nymphs. The galls open in midsummer by releasing mature aphids that infect either the same or another spruce. New galls are like green with purple or red lines, whereas the old galls are brown. Often, infested branches die, but individual trees differ in susceptibility. The eastern spruce gall adelgid can be best controlled by the spraying process.
The greenbug (which is Toxoptera graminum) is the most destructive pests of oats, wheat, and other small grains. It appears as yellow patches on the plant and can wipe out an entire field. Pale green adults contain a dark green stripe down the back. Each female produces between 50 - 60 young per one generation, and there are up to 20 generations per year. It is controlled by insecticides and parasites.
The green peach aphid (which is Myzus persicae) is also known as the spinach aphid, is a pale yellow-green having three dark lines on the back. Their life cycle involves two hosts. The female parthenogenetically reproduces during the summer and produces sexual males and females in autumn. It is a serious pest, transmitting several plant mosaic diseases.
The cotton, or melon, aphid (which is Aphis gossypii) is green to black. In cooler areas, there is an egg stage, while in warm climates, live young are produced in the whole year. Among the dozens of possible hosts are cotton, melon, and cucumber. Usually, it is naturally controlled by occurring predators and parasites (controlling aphids).
The pea aphid (which is called Acyrthosiphon Pisum) contains two colour morphs, pinkish-red and pale green. It overwinters on alfalfa and clover, migrating to peas in the spring season. Often, the yellow bean mosaic virus that it transmits is responsible for pea plant killing. Every female produces 50 - 100 young in each of 7 - 20 generations in a year. It can be controlled by weather conditions and insecticides.
Also, it is susceptible to natural predators such as ladybugs and the parasites such as acarid Allothrombium pulvinum. The colour of pea aphids, which is determined by variations in genes that produce the carotenoid pigments, may help them evade parasites and predators. Its ability to produce carotenoids is the result of a process called horizontal gene transfer, where the pea aphid acquired carotenoid genes from fungi tens of millions of years ago. The production of carotenoids has been linked to energy (ATP; adenosine triphosphate) production in the pea aphids.
The potato aphid (which is called Macrosiphum euphorbiae) begins as black eggs on the rose plants (aphids on roses) that hatch into green and pink young that feed on leaves and rosebuds. In the early spring season, they migrate to potatoes, which are the summer hosts. One generation takes place every 2 - 3 weeks. It is the carrier of potato and tomato mosaic virus diseases that kill blossoms and vines.
The rose aphid (which is called Macrosiphum rosae) is large, green with pink markings and black appendages. It is common on its only host, which is called a cultivated rose. Natural predators are aphid lions (lacewing larvae) and ladybird larvae.
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The rosy apple aphid (which is called Dysaphis plantaginea) deforms fruit, producing "aphis apples." Its feeding activity causes the leaves to curl about it, providing some sort of protection from insecticide sprays. This life cycle involves plantain plants as alternate hosts, where the aphid returns to the apple tree to deposit the eggs during the fall. Also, it attacks pear, mountain ash and hawthorn. Often, it is controlled by the natural enemies, chiefly lady beetles, syrphid flies, parasitic wasps and lacewings. However, sometimes insecticides may be necessary.
The woolly apple aphid (which is called Eriosoma lanigerum) lives on roots and may kill or stunt apple trees. White cottony masses (of white aphids) will enclose the young aphids and are controlled by parasites.
Aphids, including the closely related phylloxerans and adelgids, probably evolved from a common ancestor around 280 million years ago, in the period of Early Permian. They probably fed on plants like Cycadophyta or Cordaitales. Aphids do not fossilize well with their soft bodies, and the oldest well-known fossil is of the species Triassoaphis cubitus from the Triassic. However, they do sometimes get stuck in plant exudates that solidify into amber. When Professor "Ole Heie" produced his monograph Studies on the Fossil Aphids in 1967, up to sixty species from the Jurassic, Triassic, Cretaceous, and mainly Tertiary periods had been defined, with Baltic amber supplying the other forty species.
The whole count of species was small but considerably increased with the appearance of the angiosperms around 160 million years ago, as this has allowed aphids to specialize, the speciation of aphids going hand-in-hand with the flowering plants' diversification. The earliest aphids were probably polyphagous, having monophagy developing later. It has been hypothesized that the ancestors of the Adelgidae lived on the conifers while those of the Aphididae fed either on the sap of Araucariaceae or Podocarpaceae that survived extinctions in the Late Cretaceous.
Organs like the cornicles did not appear till the completion of the Cretaceous period. Alternatively, a study suggests that ancestral aphids might have lived on angiosperm bark and which feeding on leaves can be a derived trait. The Lachninae have long mouthparts that are suitable for living on the bark and it has been suggested that the mid-Cretaceous ancestor fed on the bark of angiosperm trees, switching to the leaves of conifer hosts in the late Cretaceous. The Phylloxeridae can well be the oldest family still extant, whereas their fossil record is limited to the Lower Miocene Palaeophylloxera.
Interactions with Humans
Up to 5000 species of aphid have been defined and of these, a few 450 species have colonized fiber and food crops. As direct feeders on the plant sap, they damage crops and reduce yields, whereas they hold a greater impact by being vectors of the plant viruses. The transmission of these viruses will depend upon the movements of aphids between nearby plants, different parts of a plant, and further afield. In this particular respect, the probing behaviour of an aphid by tasting a host is more damaging compared to the lengthy aphid feeding and reproduction by the stay-put individuals. The aphid's movement influences the timing of virus epidemics.