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).

Sections in this article:

• Introduction
• Classification
• Damage to Plants
• Ant Cows

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

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.

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.

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