Introduction

A central issue in ecology is to identify the mechanisms driving and maintaining community diversity. Studies of plant–insect associations have played an important role in understanding ecological and evolutionary processes that underlie community dynamics (Whitham et al., 2006; Poelman et al., 2008b). Plants are autotrophic organisms that produce organic matter from carbon dioxide, water and sunlight, and as such they are at the base of most food webs. There is a wealth of animals that feed on plants, and insects are by far the most speciose of these. There are an estimated 6 million insect species, of which half are herbivorous (Schoonhoven et al., 2005). Terrestrial plant–animal communities, therefore, represent a large proportion of the communities on Earth.

Plants produce a multitude of organic compounds ranging from simple molecules such as ethylene and methanol to complex terpenoids and nitrogen-containing alkaloids. More than 100 000 chemical products are known to be produced by plants (Schoonhoven et al., 2005) with estimates ranging up to 200 000 (Pichersky & Gang, 2000), and these compounds affect many interactions with community members. Recent studies have provided ample information on the molecular basis and ecology of plant defences against insects (Kessler & Baldwin, 2002; Arimura et al., 2005; Dicke et al., 2009). Constitutive defences of plants differentially affect various insect herbivores. These defences can affect the behaviour of herbivores during host-plant selection and their performance after plant tissues have been ingested (Kessler et al., 2004; Schoonhoven et al., 2005).