Coevolution of pierid butterflies and their cruciferous foodplants : V. Habitat selection, community structure and speciation.

Early models of hostplant exploitation by phytophagous insects suffer from unwarranted assumptions and may not be generally applicable. Wordmodels of the co-evolutionary approach may assume unwarranted evolutionary stability in 'strategic' explanations, whilst mathematical models derived from earlier optimal-diet studies are unrealistic and unwieldy. A simple arithmetic model synthesises these two approaches, using the two parameters of foodplant suitability and availability. Hostplant use by the butterfly Anthocharis cardamines, previously thought to be maladaptively polyphagous, is shown to be optimal under prevailing conditions of short search time. The predictions of the model for hostplant use and community structure of butterflies and other phytophagous insects are tested and, in large part, corroborated. Monophagy and monophagic forms of oligophagy are shown to be favoured by: long adult lifespan; low search costs to females; search images (Whether visual or olfactory); batch-laying of eggs; high differential in foodplant suitability.