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aphid

aphid or plant louse, tiny, usually green, soft-bodied, pear-shaped insect injurious to vegetation. It is also called greenfly and blight. Aphids are mostly under 1/4 in. (6 mm) long. Some are wingless; others have two pairs of transparent or colored wings, the front pair longer than the hind pair. In typical aphids (family Aphididae), two tubes called cornicles project from the rear of the abdomen and exude protective substances. Aphids feed by inserting their beaks into stems, leaves, or roots, and sucking the plant juices. Usually they gather in large colonies.

The life cycle of aphids is complex and varies in different species. In a typical life cycle, several generations of wingless females, which reproduce asexually (see parthenogenesis) and bear live offspring, are followed by a generation of winged females, which bears a sexually reproducing, egg-laying generation of males and females. Mating usually occurs in fall, and the eggs are laid in crevices of the twigs of the host plant; the first generation of wingless females hatches in spring. Different host plants and different parts of the plant may be used at different stages of the life cycle.

Some aphids (e.g., the woolly apple aphid) secrete long strands of waxy material from wax glands, forming a conspicuous woolly coating for their colonies. Gall-making aphids live in galls, or swellings of plant tissue, formed by the plant as a reaction to substances secreted by the insects; galls of different aphid species are easily identified (e.g., the cockscomb gall of elm leaves). One group of aphids lives only on conifers (e.g., the eastern spruce gall aphid).

Ant Cows

Many kinds of aphid secrete a sweet substance called honeydew, prized as food by ants, flies, and bees. This substance consists of partially digested, highly concentrated plant sap and other wastes, and is excreted from the anus, often in copious amounts. Certain aphid species have a symbiotic relationship with various species of ants that resembles the relationship of domestic cattle to humans; hence the name "ant cows" for aphids. The ants tend the aphids, transporting them to their food plants at the appropriate stages of the aphids' life cycle and sheltering the aphid eggs in their nests during the winter. The aphids, in turn, provide honeydew for the ants.

Damage to Plants

The damage done by aphids is due to a number of causes, including loss of sap, clogging of leaf surfaces with honeydew, and growth of molds and fungi on the honeydew. Leaf curl, a common symptom of aphid infestation, occurs when a colony attacks the underside of a leaf, causing its desiccation. The downward curl provides protection for the colony, but the leaf becomes useless to the plant. Some species also transmit viral diseases of plants. Among the aphids causing serious damage to food crops are the grain, cabbage, cornroot, apple, woolly apple, and hickory aphids and the alder and beech tree blights. The phylloxera, notorious for its damage to vineyards, is closely related to the aphids.

Many larger insects that feed on aphids, such as ladybird beetles and lacewings, are used as biological controls of aphid infestations. Fungal infection and damp weather also help limit the number of aphids.

Classification

Aphids are classified in several families of the phylum Arthropoda, class Insecta, order Homoptera.

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Aphididae

Aphididae (aphids, greenfly, blackfly; order Hemiptera, suborder Homoptera) Family of soft-bodied insects that feed on plant sap using the rostrum that arises between the front pair of legs. Typically, one generation a year reproduces sexually, laying overwintering eggs, usually on woody plants, and several generations reproduce asexually and viviparously, either on the woody host or on herbs. Many species are tended by ants which feed on the honeydew. Some are serious pests. There are about 4000 species, occurring mainly in the temperate northern hemisphere.

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aphid

aphid (plant louse) Winged or wingless, soft-bodied insect found worldwide. It transmits virus diseases of plants when sucking plant juices. Females reproduce with or without mating, producing one to several generations annually. Common species are also known as blackfly and greenfly. Length: to 5mm (0.2in). Family Aphididae.

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aphid

a·phid / ˈāfid; ˈaf-/ • n. a minute bug (superfamily Aphidoidea, suborder Homoptera) that feeds by sucking sap from plants.

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aphids

aphids See APHIDIDAE.

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aphid

aphidplaided, unpadded •backhanded, candid, candied, heavy-handed, high-handed, offhanded, red-handed, short-handed, unbranded, underhanded •retarded, unregarded •bareheaded, boneheaded, fatheaded, hard-headed, hot-headed, light-headed, pig-headed, pinheaded, thickheaded, unleaded, unwedded, wooden-headed, wrong-headed •intended, splendid, unamended, unapprehended, unattended, unblended, undefended, untended •gadid, unaided, unpersuaded, unshaded •reeded, unheeded, unimpeded, unneeded, unseeded •unshielded • katydid •lopsided, misguided, one-sided, undecided, undivided, unguided, unprovided •broadminded, like-minded, simple-minded, single-minded, small-minded, tough-minded •disembodied •sordid, unrecorded, unrewarded •unclouded, uncrowded •unbounded, unfounded, ungrounded •outmoded, spring-loaded, unexploded •unwounded •unhooded, wooded •cold-blooded, hot-blooded, red-blooded, unstudied, warm-blooded •underfunded, unfunded •unheralded • aphid • triffid •jagged, ragged •cross-legged, legged •dogged • rugged

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Aphids

Aphids

Reproductive habits

Ants and aphids

Resources

Aphids are insects in the order Homoptera, also known as plant lice. Some 3, 800 aphid species have been identified worldwide with 1, 300 species found in North America, which includes some 80 species classified as pests of crops and ornamental plants. Aphids have a distinctive pear-shaped body, and most are soft and green in color. The wings are transparent and held in a tent-like position over the abdomen, which has a short tail, called a cauda. The legs are long and thin, as are the six-segmented antennae. Two tube-like structures, called cornicles, project from the fifth or sixth abdominal segment and excrete a defensive chemical when the aphid is threatened.

Reproductive habits

Aphids have a complicated life cycle and reproduction habits that make them extremely adaptable to their host plants and environments. When aphid eggs

that have overwintered on their host plants hatch in the spring, they produce females without wings. These females are capable of reproducing asexually, a process called parthenogenesis. Several asexual generations may be produced during a growing season.

When it becomes necessary to move to another plant, winged females are produced. As winter approaches, both males and females are produced and their fertilized eggs again overwinter until the next spring. Sometimes winged females that produce asexually also migrate to new hosts. The lack of wings among generations of aphids that have no need to migrate is seen as an adaptive advantage, since it helps keep them from being blown away in windy weather.

Ants and aphids

Ants and aphids A symbiotic relationship exists between ants and aphids. They are often compared to cattle, with the ants acting as protectors and ranchers. Aphids secrete a sweet substance called honeydew, which contains surplus sugar from their diet. Ants protect aphid eggs during the winter, and carry newly hatched aphids to new host plants, where the aphids feed on the leaves and the ants get a supply of honeydew.

Because they reproduce rapidly and grow large colonies, an aphid infestation stunts growth, inhibits the crop production, and can even kill the host plant. Aphids can also carry other diseases, such as viruses, from one plant to another. Their saliva is also toxic to plant tissues. Among the biological controls of aphid infestations in agriculture and horticulture are lacewings, sometimes called aphid lions, lady beetles or ladybird beetles (ladybugs), and syrphid flies. Pesticides, including diazinon, disyston, malathion, nicotine sulfate, and others, are also used to control aphids. On a smaller scale, some gardeners control aphids by simply washing them off with a spray of soapy water.

KEY TERMS

Honeydew A sweet substance excreted by aphids that ants utilize as food.

Parthenogenesis Asexual reproduction without the fertilization of eggs.

Symbiosis A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.

Resources

BOOKS

Arnett, Ross H. American Insects. New York: CRC Publishing, 2000.

Hubbell, Sue. Broadsides from the Other Orders: A Book of Bugs. New York: Random House, 1993.

Imes, Rick. The Practical Entomologist. New York: Simon & Schuster, 1992.

McGavin, George C. Bugs of the World. London: Blandford Press, 1999.

Vita Richman
Neil Cumberlidge

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Aphids

Aphids

Aphids are insects in the order Homoptera, which are also known as plant lice. Some 3,800 species of aphids have been identified worldwide with 1,300 species occurring in North America , which includes some 80 species that are pests of crops and ornamental plants. Aphids have a distinctive pear-shaped body, and most are soft and green in color . The wings are transparent and are held in a tent-like position over the abdomen, which has a short tail, called a cauda. The legs are long and thin, and the antennae are thin and have six segments. Two tube-like structures, called cornicles, project from the fifth or sixth abdominal segments of aphids. The cornicles excrete a defensive chemical when the aphid is threatened.


Reproductive habits

Aphids have a complicated life cycle and reproduction habits that make them extremely adaptable to their host plants and environments. When aphid eggs that have overwintered on their host plants hatch in the spring, they produce females without wings. These females and are capable of reproducing asexually, a process called parthenogenesis . Several asexual generations of aphids may be produced during a growing season.

When it becomes necessary to move to another plant, females with wings are produced and move to another host plant. As winter approaches, both males and females are produced and their fertilized eggs again overwinter until the next spring. Sometimes winged females that produce asexually also migrate to new hosts. The lack of wings among generations of aphids that have no need to migrate is seen as an adaptive advantage, since it helps keep them from being blown away in windy weather .


Ants and aphids

An intimate, symbiotic relationship exists between ants and aphids. They are often compared to cattle, with the ants acting as protectors and ranchers. What aphids have that ants want is something called honeydew, a sweet substance that is excreted by aphids through their anus and contains surplus sugar from the aphid's diet. Ants protect aphid eggs during the winter, and carry the newly hatched aphids to new host plants, where the aphids feed on the leaves and the ants get a supply of honeydew.

Because of their ability to reproduce rapidly and grow large colonies, their feeding on plants causes yellowing, stunting, mottling, browning, and curling of leaves, as well as inhibiting the ability of the host plant to produce crops. Infestations by aphids can cause plants to die, and the insects can carry other diseases, such as plant viruses, from one plant to another. Their saliva is also toxic to plant tissues. Among the biological controls of aphid infestations in agriculture and horticulture are lacewings , sometimes called "aphid lions," lady beetles or ladybird beetles (ladybugs), and syrphid flies . Pesticides , including diazinon, disyston, malathion, nicotine sulfate, and others, are also used to control aphids. On a smaller scale, some gardeners control aphids by simply washing them off with a spray of soapy water .

Resources

books

Arnett, Ross H. American Insects. New York: CRC Publishing, 2000.

Hubbell, Sue. Broadsides from the Other Orders: A Book ofBugs. New York: Random House, 1993.

Imes, Rick. The Practical Entomologist. New York: Simon & Schuster, 1992.

McGavin, George C. Bugs of the World. Blandford Press, 1999.


Vita Richman Neil Cumberlidge

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Exoskeleton

—A hard, shell-like structure that serves both to protect the vital organs of animals without an internal skeleton and to support their muscle systems.

Honeydew

—A sweet substance excreted by aphids that ants need.

Parthenogenesis

—Asexual reproduction without the fertilization of eggs.

Spiracles

—Openings that lead to a system of tubes that supply air to insects.

Symbiosis

—A biological relationship between two or more organisms that is mutually beneficial. The relationship is obligate, meaning that the partners cannot successfully live apart in nature.

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