Aphids

Abstract

The aphids are soft-bodied small insects (< 7 mm) and feed by sucking plant sap. They usually live in colonies on the undersides of leaves or tender terminal shoots. Aphids excrete considerable amount of sugary liquid, honeydew, on which sooty mould usually turns them black and also serves as food for ants, bees and their parasitic wasps. The aphids are unique on the account of their peculiar mode of reproduction, development and polymorphism. They may reproduce either by parthenogenesis, zygogenesis or paedogenesis. They may either be oviparous or viviparous and alatae or apterae, the male often being wanting and frequently rare in certain generations. Parthenogenetic reproduction allows rapid increase in numbers and results in populations consisting of clones. Some species reproduce both parthenogenetically and sexually (holocyclic species), whereas only few reproduce parthenogenetically (anholocyclic species). In parthenogenetic reproduction, life cycle completes within 10 days in temperate regions. The aphids are polymorphic, and both winged (alate) and wingless (aptera) morphs may be found in the same colony. Several factors, both biotic and abiotic, have effect on the formation of different phenotypes. Each morph performs different ecological roles in the life history which is characteristic of aphids. This trait coupled with the ability to breed by means of diploid parthenogenesis and viviparity for a major part of the life cycle in aphids has enabled them to produce a large number of clones in different kinds of plants even under adverse conditions. Aphids are frequently engaged in mutualistic associations with bacterial endosymbionts that not only provide essential amino acids to them but also grant them protection from natural enemies, protection from extreme temperatures, development of resistance to a fungal pathogen and the ability to use a greater diversity of resources. Out of globally 5110 species of aphids described, about 250 species are major agricultural and horticultural pests. They damage the crops directly by sucking their nutrients, making galls and hampering photosynthesis and respiration by the growth of sooty moulds on the honeydew deposited thereon. Aphids also damage the crop indirectly by transmitting hundreds of plant viruses. Because of their economic importance, their population must be controlled to save the crops. In this contribution, several aspects of aphid systematic and biology such as endemism, host–plant association, diversity, morphology, feeding behaviour, life history, polymorphism and factors affecting it, migration, defence, aphid–ant association, endosymbiosis, economic importance and their population management have been described in detail.