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Foraging scent-marks in bees

Insect epicuticular lipids are known to serve a range of functions, including the reduction of evaporative water loss over the body surface as well as improving adhesion for movement on smooth surfaces. Additionally, the same lipids may constitute chemical signals for conspecifics, e.g mediate mate recognition or relate information on reproductive status. In foraging bees, especially bumblebees (Bombus sp.), epicuticular lipids are thought to function as scent marks, that allow foragers to detect and avoid flowers that have been previously visited and depleted by themselves or other individuals (Goulson et al. 1998; Gawleta et al. 2005). When approaching an inflorescence bumblebee workers can often been seen to briefly hover in front of a given flower, but then reject it without actually probing for nectar. As the rejected flowers contain on average less nectar than probed flowers, discrimination minimizes search time and energy expenditure. Behavioral experiments by D. Goulson and coworkers have indicated that the remote assessment of flowers is not based on a direct perception of nectar rewards but rather indirectly on the perception of hydrocarbon marks deposited by previous visitors. Flowers treated with hexane extracts of bumblebee tarsi elicited a repellent effect similar to that of a natural bumblebee visit, with nectar levels remaining unaltered. Similar results were obtained when applying to flowers several pure synthetic hydrocarbons (n-alkanes, alkenes), which had been previously shown to be the main contents of epicuticular lipids (Goulson et al. 2000).

It remained unclear whether the deposition of repellent substances is an active process that merits the term 'scent marking' or whether the deposits are mere unavoidable 'footprints'. In a laboratory experiement we demonstrated that feeders that had been walked over on the way from the nest to the foraging arena were equally repellent to foraging bumblebees as feeders that had been walked over during an actual flower visit. This suggests that the deposited substances are footprint cues rather than pheromone signals (Wilms & Eltz, 2008).

As a further complication the repellent effect is only temporary. Normally, the repellency of visited flowers is lost after 30 to 60 minutes depending on flower species/rate of nectar secretion (Stout and Goulson 2001). This makes sense given that nectar is replenished at corresponding rates. However, given that the deposited cuticular hydrocarbons are all of very low volatility, it is unclear what mechanism mediates the rapid decline of footprint repellency. We are currently investigating two possible hypotheses: (1) The low volatility lipoid substances are absorbed by the epicuticular wax of the corolla surface and are no longer perceptible by the foraging bumblebees, or (2) other, yet undicovered, trace volatiles are present in footprints that mediate repellency.

The low volatility of footprint hydrocarbons has a convenient corollary for pollination ecologists. It is possible to extract bumblebee hydrocarbons from flowers at the end of the day, quantify them by gas chromatography, and use this data to estimate the cumulative number of visits a flower has recieved over its lifetime. We have validated such an approach in a natural population of comfrey, Symphytum officinale (Witjes & Eltz 2009). Consecutive projects have revealed that it is also possible to reconstruct bumblebee visitor communities (species composition)of a comfrey plant based on the hydrocarbon footprint profiles on its flowers, and that the amount of footprints deposited on flowers is a predictor of seed set in this self-incompatible plant species (Witjes et al. 2011). We are currently investigating the potential of "extractive footprint quantification" for other groups of pollinators, especially the honeybee (Apis mellifera).

References:
Stout, J. C., Goulson, D. & Allen, J. A. 1998. Repellent scent-marking of flowers by a guild of foraging bumblebees (Bombus spp.). Behavioral Ecology and Sociobiology 43, 317-326.

Goulson, D., Stout, J. C., Langley, J. & Hughes, W. O. H. 2000. Identity and function of scent marks deposited by foraging bumblebees. Journal of Chemical Ecology 26, 2897-2911.

Stout, J. C. & Goulson, D. 2001. The influence of nectar secretion rates on the responses of bumblebees (Bombus spp.) to previously visited flowers. Behavioral Ecology and Sociobiology 52, 239-246.

Gawleta, N., Zimmermann, Y. & Eltz, T. 2005. Repellent foraging scent recognition across bee families. Apidologie 36, 325-330.

Eltz T., 2006. Tracing pollinator footprints on natural flowers. Journal of Chemical Ecology 32, 907-915.

Witjes, S. & Eltz, T. 2007. Influence of scent deposits on flower choice: experiments in an artificial flower array with bumblebees. Apidologie 38, 12-18.

Wilms, J. & Eltz, T. 2008. Foraging scent marks of bumblebees: footprint cues rather than pheromone signals. Naturwissenschaften 95: 149-153. PDF

Witjes S. & Eltz T. (2009) Hydrocarbon footprints as a record of bumblebee flower visitation. Journal of Chemical Ecology 35:1320-1325. PDF

Witjes S., Witsch,K. & Eltz, T. (2011) Reconstructing the pollinator community and predicting seed set from hydrocarbon footprints on flowers. Oecologia 165:1017-1029. PDF
long-tongued Bombus pascuorum visiting field bean