Aphids life cycle
Aphids have a complex life cycle, with both winged and wingless forms of adults of the same species developing, depending on the conditions. Wingless adult aphids are known as apterous, and winged adults as alate. Alate aphids have two pairs of wings, one of which is much larger than the other.
For a large part of the season an aphid population consists of viviparous females. As reproduction is asexual, the progeny of a single female is genetically identical to the mother. In other words, they are clones. Because there is no genetic recombination, different characteristics, such as colour forms, or resistance to pesticides, remain unchanged and unsorted. The young aphids are born as developed nymphs and begin to feed instantly on plant sap. They grow rapidly and moult four times before becoming adult, with the conspicuous white cuticles, shed during each of these moults, betraying their presence in the crop.
There are two types of aphid species: those that change to other host plants in the winter, and those species that do not. Species that alternate between host plants reproduce asexually on their summer host plant, migrating to their winter host plant in autumn, where they reproduce sexually and lay eggs that overwinter. The summer host plants are either herbaceous or woody, while the winter host plants are hardy, woody perennials. Aphids that do not alternate between host plants also mate in the autumn and lay eggs which overwinter. Where mating occurs and eggs are laid, the life cycle is termed holocyclic (complete). In greenhouses, however, the alternation of host plant and egg-laying may not take place. In this case, reproduction continues through the winter by parthenogenesis, with viviparous unfertilized females continuing to produce new generations of females. This is known as an anholocyclic (incomplete) life cycle. However, in some greenhouse crops with very low winter temperatures (e.g. strawberries), aphid eggs have been observed.
