Introduction

The incorporation of plant diversity within agricultural systems has led to decreased insect pest densities in approximately 50% of studies in which monocultures and polycultures were directly compared (Risch et al. 1983; Andow 1991; Coll 1998; Gurr et al. 2000). One of the leading hypotheses explaining the observation of decreased pest densities under polycultures is that increased plant diversity can enhance the action of natural enemies of pests (the “enemies hypothesis” of Root 1973). Increased plant diversity can provide natural enemies with resources such as a favorable microclimate, alternative hosts or prey, or plant-based foods such as pollen, nectar, or honeydew (Landis et al. 2000). In this chapter, we focus on one of the more intuitively clear predictions encompassed within Root's enemies hypothesis – the idea that the presence of nectar-producing plants can improve biological control of pests by supplying parasitoids with sugar. Note that this idea includes two components: an outcome (improved biological control) and an underlying mechanism (nectar-feeding), both of which need to be demonstrated. We refer to the combined outcome and mechanism as the “parasitoid nectar provision hypothesis”.

The hypothesis that plant diversification can decrease pest pressure by providing sugar to parasitoids that would otherwise be sugar-limited has its origins in anecdotal or semi-quantitative observations of increased parasitism rates and biological control in the vicinity of flowering plants.