**2. Aphid biology and behavior**

Aphids are specialized phloem sap feeders and chemists *par excellence*. In most of cases, they exhibit passive feeding by high pressure within the sieve elements (SEs) and feed on virtually all plant families. While most of the species are specialists on a single host plant, some of them are generalists with relatively broad host range [5]. The aphid life cycles involve sexual and asexual morphs, and most of the species have relatively complicated life cycles with morphs that specialize in reproduction, dispersal, and survival under adverse conditions. Based on host utilization, aphids have two different types of life cycle: heteroecious or host alternating and monoecious/autoecious or nonhost alternating. Heteroecious species live on one plant species (primary host) in winter and migrate to another taxonomically unrelated plant species (secondary host) in summer and again migrate to primary host in autumn. While oviparity is exhibited on the primary host, on the secondary host, they reproduce parthenogenetically. These changes in sexual fate and reproductive mode are condition dependent and explain the extraordinary plasticity in development in response to environmental cues. Aphid species that interrupt parthenogenetic reproduction with sexual reproduction are termed as holocyclic. In contrast to hostalternating aphids, nonhost-alternating aphids remain either on the same or closely related host species throughout the year. They complete both sexual life cycle as well as parthenogenetic life cycle on the same host species. In contrast to this, there are species which do not produce eggs and are known as anholocyclic. Some species, particularly those having cosmopolitan distribution, exhibit both holocyclic and anholocyclic life, both at the same time in different geographical areas [6] but rarely both monoecy and heteroecy [7]. The presence of both biparental sexual and asexual life cycle ensures that aphids take advantage of both genetic recombination that help them to evolve and parthenogenesis (very convenient to exploit short-lived hosts).
