Frequency-dependent host choice by phytophagous insects and the evolution of plant defence strategies

Abstract

Herbivore adaptation to plant resistance presents an evolutionary trade-off for plants. In the absence of plant resistance the plant is susceptible to non-adapted herbivores. However herbivores that are adapted to a particular plant resistance trait may increase the cost to the plant of expressing the trait if the herbivores use it as an attractant. The adapted herbivore escape hypothesis suggests that plants may get around this problem by only deploying the resistance when necessary in response to herbivore feeding. This hypothesis requires that adapted herbivore are able to discriminate in favour of resistant plants when making host choice decisions and that host selection is frequency-dependent such that the undamaged phenotype can escape when rare. This thesis set out to determine how host phenotype frequency affects choice by insect herbivores in both a mechanistic and a functional sense. In addition, a study was made of the conditions under which induced strategies may be evolutionary stable when under selection by adapted and non-adapted herbivores in the absence of physiological costs of defence. The role of learning and host-deprivation in the oviposition acceptance behaviour of the diamondback moth (Plutella xylostella L. (Lepidoptera: Yponomeutridae)) was investigated. Previous experience with a host plant increased acceptance in a host-specific way. Discrimination between host plants was independent of deprivation. Existing models of insect oviposition acceptance do not adequately explain these findings. A new model- The Incremental Acceptance Model- is proposed. In laboratory trials containing mixtures of damaged and undamaged Brassica oleraceae L. and B. napus L. plants at different frequencies, a key assumption of the adapted herbivore escape hypothesis was supported. The probability of oviposition by the diamondback moth was dependent on the frequency-of the damaged state and undamaged Brassica plants were more likely to escape herbivory than were damaged plants. The larval rearing plant affected the oviposition preference of the moths for damaged plants. The evolution of plant defence strategies was modelled under frequency-dependent selection by nonadapted and adapted, or partially adapted herbivores. A stable state always arose that contained both the defended and undefended plant phenotypes, except under certain restrictive conditions. Inducibility formed a pure ESS provided the defended phenotype was not fitter than the undefended phenotype and the plant defence reduced the feeding damage by non-adapted herbivores. In contrast with previous models, induction is not necessarily favoured when herbivory is predictable or the costs of expressing defence intermediate. It is argued that frequency-dependent herbivory is a major omission from previous studies of the evolution of plant defence strategies.