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One of a group of mostly soft-bodied plant-feeding insects of the suborder Homoptera, superfamily Aphidoidea. The worldwide fauna of over 4000 species is most abundant in north temperate regions. Aphids feed on phloem sap from vascular plants, tapping it through a feeding tube formed from modified mandibles and maxillae called stylets. In so doing they may transmit viruses from plant to plant, spreading serious disease in crops such as potatoes, cereals, sugarbeet, and citrus. Plants sometimes react to aphid feeding by forming galls in which the aphids live protected from drought and enemies. The so-called Chinese gall is valued in commerce for its high tannin content.

Aphids have evolved complex life styles to exploit the changing growth phases of plants. Many divide their yearly cycle by flying between a primary host, on which sexual forms mate and lay winter eggs, and a secondary host, where only parthenogenetic females multiply. Only one generation of males and sexual oviparous females occurs each year, usually in autumn. Most parthenogenetic females are also viviparous, and reproduce very rapidly under favorable conditions. Viviparae are winged or wingless . Development of young aphids can be switched toward either wingedness or sexuality by outside factors, such as crowding, decreasing temperature, or shortening days. Aphids in the tropics often remain wholly parthenogenetic.

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
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