Foraging scent marks of bumblebees: footprint cues rather than pheromone signals

Abstract

In their natural habitat foraging bumblebees refuse to land on and probe flowers that have been recently visited (and depleted) by themselves, conspecifics or other bees, which increases their overall rate of nectar intake. This avoidance is often based on recognition of scent marks deposited by previous visitors. While the term 'scent mark' implies active labelling, it is an open question whether the repellent chemicals are pheromones actively and specifically released during flower visits, or mere footprints deposited unspecifically wherever bees walk. To distinguish between the two possibilities, we presented worker bumblebees (Bombus terrestris) with three types of feeders in a laboratory experiment: unvisited control feeders, passive feeders with a corolla that the bee had walked over on its way from the nest (with unspecific footprints), and active feeders, which the bee had just visited and depleted, but which were immediately refilled with sugar-water (potentially with specific scent marks). Bumblebees rejected both active and passive feeders more frequently than unvisited controls. The rate of rejection of passive feeders was only slightly lower than that of active feeders, and this difference vanished completely when passive corollas were walked over repeatedly on the way from the nest. Thus, mere footprints were sufficient to emulate the repellent effect of an actual feeder visit. In confirmation, glass slides on which bumblebees had walked on near the nest entrance accumulated hydrocarbons (alkanes and alkenes, C23 to C31), which had previously been shown to elicit repellency in flower choice experiments. We conclude that repellent scent marks are mere footprints, which foraging bees avoid when they encounter them in a foraging context.

Social scent marks do not improve avoidance of parasites in foraging bumblebees

“…Recently, it has been shown that naive bees have no preference, neither for flowers already visited nor for those unvisited (Leadbeater and Chittka, 2011). Scent marks are mainly composed of cuticular hydrocarbons, and they correspond to footprint cues rather than pheromone signals (Goulson et al, 2000;Saleh et al, 2007;Wilms and Eltz, 2008;Witjes and Eltz, 2009). These substances are non-volatile and even tiny differences in their quantities -which accumulate on the flower after each visit and remain unchanged over a period of 24h -are detectable by social insects (D'Ettorre, 2008;Saleh et al, 2007;Witjes and Eltz, 2009).…”

The origin and dynamic evolution of chemical information transfer

“…If their reaction to the cue is selectively neutral to the sender, no evolution towards increased effectiveness of communication is expected (e.g. [26,27]). Because incidental transmission of chemical information presents the simplest and arguably the most widespread form of information transfer it should serve as a null hypothesis against which to compare more complex, evolved communication systems.…”

The pretarsus of the honeybee

“…Also important was the fact that we did not find any evidence for the Arnhart's tarsal gland to be involved in the release of the predicted footprint pheromones as suggested for the bumblebee (Bombus terrestris) (WiLms & eLtz 2008). The fact that we did not observe secretory pores either in the cuticle of the arolium, or in the cuticle of the fifth tarsomere is in agreement with the absence of such pores in honeybees (Lensky et al 1985) and wasps (BiLLen 1986), although JArAu et al (2005) did report the presence of footprint secretions in the stingless bee (Melipona seminigra Friese).…”

“…In 2003, as part of an extensive electron microscopic study, goodmAn (2003) was the first to provide a general description of the pretarsal functional morphology. In addition, JArAu et al (2004) and WiLms & eLtz (2008) proposed the occurrence of epithelial glands around the tendon to mark food sources. In order to contribute to the elucidation of the functional morphology of the pretarsal structures, we conducted an in-depth scanning electron microscopic study on these complex structures.…”