Impact of odorants on perception of sweetness by honey bees

Abstract

Organic volatiles produced by fruits can result in overestimation of sweetness by humans, but it is unknown if a comparable phenomenon occurs in other species. Honey bees collect nectar of varying sweetness at different flowering plants. Bees discriminate sugar concentration and generally prefer higher concentrations; they encounter floral volatiles as they collect nectar, suggesting that they, like humans, could be susceptible to sweetness enhancement by odorant. In this study, limonene, linalool, geraniol, and 6-methyl-5-hepten-2-ol were tested for their ability to alter behaviors related to perception of sweetness by honey bees. Honey bees were tested in the laboratory using proboscis extension response-based assays and in the field using feeder-based assays. In the laboratory assays, 6-methyl-5-hepten-2-ol and geraniol, but neither linalool nor limonene, significantly increased responses to low concentrations of sucrose compared with no odorant conditions in 15-day and 25-day-old adult worker honey bees, but not in 35-day-old bees. Limonene reduced responding in 15-day-old bees, but not in the older bees. There was no odorant-based difference in performance in field assays comparing geraniol and limonene with a no odorant control. The interaction of the tested plant volatiles with sucrose concentration revealed in laboratory testing is therefore unlikely to be a major determinant of nectar choice by honey bees foraging under natural conditions. Because geraniol is a component of honey bee Nasonov gland pheromone as well as a floral volatile, its impact on responses in the laboratory may reflect conveyance of different information than the other odorants tested.

Fig 1. Proboscis extension responses to sucrose solutions in the presence of no odorant, limonene, linalool, geraniol, or 6MH.

Data are summarized as sucrose response scores (maximum score, 5). Plots show median and interquartile ranges for honey bees tested at different ages. Results of post hoc comparisons (Dunn’s tests): *, p < 0.05; ***, p < 0.001. Sample sizes given in Table 1. (A) 15-day-old worker honey bees. (B) 25-day-old worker honey bees. (C) 35-day-old worker honey bees.

Fig 2. Concentration-dependent effect of odorants on proportion of honey bees responding to sucrose solutions in the presence of no odorant, limonene, linalool, geraniol, or 6MH.

(A) 15-day-old worker honey bees. No significant differences were observed. (B) 25-day-old worker honey bees. Significant differences were observed at 10% sucrose between 6MH and no odorant (p < 0.01); geraniol and no odorant (p < 0.02); and 6MH and linalool (p < 0.05). The difference between 6MH and no odorant was also significant at 20% sucrose (p < 0.02). (C) 35-day-old worker honey bees. There was a significant difference between 6MH and limonene at 10% sucrose only (p < 0.05). (D) All ages combined. No significant differences. Sample sizes given in Table 1.

Fig 3. No impact of limonene or geraniol on gustatory acceptance in the field.

GAS = gustatory acceptance score. Honey bees in the Low category have the greatest sucrose sensitivity, as they will return to feeders offering the lowest concentrations of sucrose. Low concentrations were 0.1% and 0.3% sucrose; Medium concentrations were 1% and 3% sucrose; High concentrations were 10% and 30% sucrose).

Fig 4. No impact of limonene or geraniol on cumulative drinking duration (seconds) during a 15-min field observation.

Testing was conducted using a sucrose concentration of 0.3 mol L^-1 (10%). Sample sizes given in parentheses. Each point represents an individual forager.