Abstract

In addition to sugars, nectar contains multiple nutrient compounds in varying concentrations, yet little is known of their effect on the reward properties of nectar and the resulting implications for insect behaviour. We examined the pre-ingestive responses of honeybees to sucrose solutions containing a mix of pollen compounds, the amino acids proline or phenylalanine, or known distasteful substances, quinine and salt. We predicted that in taste and learning assays, bees would respond positively to the presence of nutrient compounds in a sucrose solution. However, bees’ proboscis extension responses decreased when their antennae were stimulated with pollen- or amino acid-supplemented sucrose solutions. Compared to pure sucrose, bees exhibited worse acquisition when conditioned to an odour with pollen-supplemented sucrose as the unconditioned stimulus. Such learning impairment was also observed with quinine-containing sucrose solutions. Our results suggest that bees can use their antennae to detect pollen compounds in floral nectars. Depending on the type and concentrations of compounds present, this may result in nectar being perceived as distasteful by bees, making it less effective in reinforcing the learning of floral cues. Such reward devaluation might be adaptive in cases where plants benefit from regulating the frequency of bee visitation.

Highlights

  • IntroductionJournal of Comparative Physiology A (2019) 205:333–346 how other compounds may affect the taste of nectar for pollinators, and the subsequent effects on behaviour

  • Bees are capable of assessing the value of nectar rewards offered by flowers on the basis of sugar concentration and volume (Núñez 1970), but relatively little is known as to1 3 Vol.:(0123456789)Journal of Comparative Physiology A (2019) 205:333–346 how other compounds may affect the taste of nectar for pollinators, and the subsequent effects on behaviour

  • The proportion of bees responding fell significantly between the first and fifth trial (100 mM, LSD, χ2 = 247.27, p < 0.001). Though for this group there was a significant difference between the response to pure sucrose at the beginning and end of the taste assay (T1–T6, df = 1, χ2 = 5.1, p = 0.024), close to 75% of bees exhibited proboscis extensions on the final trial. This suggests that bees were still very motivated to respond to pure sucrose, compared to the sucrose–quinine mixture (T5–T6, df = 1, χ2 = 39.55, p < 0.001)

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Summary

Introduction

Journal of Comparative Physiology A (2019) 205:333–346 how other compounds may affect the taste of nectar for pollinators, and the subsequent effects on behaviour. This question is of importance, given that nectar is more than just sugar water and frequently contains a wide variety of other compounds, of which amino acids are the most abundant (Baker and Baker 1973, 1975, 1977; Gottsberger et al 1984; Nepi 2014). Since learning plays an important role in shaping the foraging decisions of bees (Menzel 1985), the taste of nectar may have a direct impact on the number of pollinator visits a flower receives (Gardener and Gillman 2002; Petanidou et al 2006; Stevenson et al 2017). Contamination of nectar with pollen grains, dislodged during flower visits by insects, can result in a notable increase in amino acid concentration (Gottsberger et al 1990; Erhardt and Baker 1990)

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