An ecological perspective of host alternation in aphids (Homoptera ; Aphidinea : Aphididae)

Complex lifecycles, in which a regular seasonal shift in habitat occurs, have evolved in numerous taxa. Many species of aphid seasonally switch between unrelated host plants. This phenomenon, known as host alternation or heteroecy, has long attracted interest. Bonnet [ 1745] suspected it existed, and it was first described by Walker [ 1 848]. The typical life cycle involves migration between a woody host, on which sex occurs and eggs are laid (the primary host) and a herbaceous plant, where the reproduction is solely parthenogenetic (the secondary host). Clearly, the twice yearly immigration cycle with associated mortality, is a cost, especially in a taxon with limited host location abilities. Yet many aphid species are rigidly fixed to a system which would appear risky and dangerous. Why then, do they do it? Throughout the literature of the last century, 2 themes are presented as answers, (a) the adaptive hypotheses and (b) the maladaptive. The development of these ideas is reviewed, and a modern synthesis presented: It is concluded that such obligate resource switching occurs when changing hosts is predictably more profitable than remaining on the same plant species. The major cause of this benefit is where alternative sympatric hosts display asynchronous phenologies.