The ‘sweet tooth’ of the honeybee: the perception of nectar and its infl uence on honeybee behaviour
Nectar, an aqueous mixture of nutrient compounds offered by fl owering plants to attract many kinds of pollinators, is one of the main foods eaten by honeybees. Nectar is the currency of the exchange between pollinators and plants and, as such, we can
reasonably assume that its traits have been produced by natural selection exerted by pollinators (Dilcher, 2001; Grimaldi, 1999; Pacini et al., 2007). Far from being composed simply of sugars and water, nectar is a complex solution often containing many other substances (Baker and Baker, 1983), including amino acids (Gardener and Gillman, 2001), scent compounds (Raguso, 2004), salts (Hiebert and Calder, 1983), and toxins (Adler, 2000; for reviews of nectar composition, see Baker and Baker, 1983 and Nicolson et al., 2007). The role of many of the non-nutritive compounds secreted by plants into fl oral nectar is not well understood, though some proposed functions include anti-microbial agents or deterrents to nectar robbers. Indeed, the compounds that are not sugars or amino acids may have other nutritional benefi ts; nectar has also been reported to contain vitamins (Baker and Baker, 1983), carotenes (Rodriguezarce and Diaz, 1992), and lipids (Vogel, 1971). It seems counter-intuitive, however, that nectar would contain toxins, but several types of chemicals toxic to insects have been reported (for a review, see Adler, 2000) and their ill effects on pollinators, such as honeybees, have been documented (Detzel and Wink, 1992). Plants may also secrete proteins, called nectarins, into nectar. These substances act as anti-microbial agents that prevent the quality of nectar from deteriorating as a result of microbial infection (Carter and Thornberg, 2004).