Introduction Nectar is a sweet liquid produced by plants on various parts of the plant body. Most people are familiar with nectar in flowers, collected by bees to make honey, and utilized by a variety of floral visitors, some of whom serve as pollinators for the plant. Less familiar is extrafloral nectar, produced outside the flowers in extrafloral nectaries and usually not associated with pollination. Plants produce nectar in various ways (Elias 1983; Koptur 1992a), and whether they do it purposefully (secretion) or passively (excretion) has been the subject of debate between physiologists and evolutionary ecologists for many years (reviewed in Bentley 1977; see also Sabelis., Chapter 4). Over evolutionary time, myriad selective forces have shaped not only the morphology and function of nectaries, but also the composition of the substances secreted and whether or not the structures secrete under different circumstances. Thompson's (1994) synthetic theory of the “co-evolutionary mosaic”, in which different populations of a given species experience different interactions over space and time, helps to explain the variable findings researchers encounter in studying interactions between plants and predatory insects, especially those mediated by nectar (or other direct or indirect food rewards from plants). Carnivorous organisms, which can benefit plants as protectors, may rely on nectar as an energy source. If ants, wasps, other predators, and parasitoids are more likely to encounter their herbivore prey if they utilize a plant's nectar, mutualisms are thus promoted.
