Production of synthetic trail pheromone of bumblebees might well prove useful in inducing bumblebee queens to discover and occupy nest boxes placed for them in the ﬁeld, and so increase the numbers of bumblebee workers available for pollination.
Parasitic bees of the genus Psithyrus have their brood reared in bumblebee colonies. The Psithyrus females appear to use the bumblebee trail pheromo- nes to recognize and locate the particular bumblebee species they parasitize (Cederberg, 1983). When Psithyrus females were presented with methanol extracts of the body surface of B. lapidarius queens they eagerly palpated them with their antennae. Extracts of all major parts of the queen, i.e. head, thorax and abdomen were attractive, but extracts of the tarsi were especially so. P. rupestris females readily followed odour trials of B. lapidarius that were either natural or made from queen extracts. They were also able to select extracts of their B. lapidarius host from extracts of six other bumblebee species.
Trail pheromone while foraging
Bumblebees may also mark ﬂowers with pheromone. Cameron (1981) observed that bumblebee foragers probed artiﬁcial ﬂowers that had a reward (honey or sucrose syrup) to offer or that had recently provided a reward, but did not probe ﬂowers that had never offered a reward. He found that the deposit used to mark the rewarding ﬂowers was soluble in hexane or pentane, and after artiﬁcial ﬂowers had been washed with these solvents they no longer released probing. The extract has not yet been tested to ﬁnd out whether or not it releases probing when added to non-rewarding ﬂowers. The bumble- bees gyrated and groomed themselves on rewarding ﬂowers and this be- haviour probably facilitated deposition of the pheromone. On natural ﬂowers bumblebees make plentiful bodily contacts with petals, stamens and pistils, especially when scrabbling for pollen and when the ﬂowers have tubular corollas.
No experiments have been done to ﬁnd whether the message left by successful foragers is species speciﬁc. Attracting bees from other bumblebee colonies to favourable nectar sources could be detrimental. However, mem- bers of a bumblebee colony forage in close proximity to their nest, so probably many, if not most, of the ‘messages’ left on ﬂowers would reach members of the departing bumblebee’s own colony, and so help nestmates avoid ﬂowers that are in a pre- or post- nectar secreting stage and attract them to favourable patches of ﬂowers. Perhaps the pheromonal messages are most valuable to the departing bumblebee itself by indicating which ﬂowers it has already visited on the current foraging trip, and which it would pay to revisit on a subsequent one.
It is likely that bumblebees, like honeybees (page 112), use a deterrent pheromone. Visits by an individual bumblebee forager to ﬂowers on its ‘foraging circuit’ are in such a time sequence as to maximize the reward from each ﬂower visit (Corbett et al., 1984). Flowers probed during the past minute or so are either not landed on, or landed on but not probed. The source of such a deterrent pheromone is unknown.
For both bumblebees and honeybees the extent to which the pheromone messages are heeded must depend on the availability of forage and competi- tion from other bees. When competition is ﬁerce individual ﬂowers are visited at very frequent intervals, and a bee may attempt to land on a ﬂower while another is still present. Clearly under such conditions pheromone messages, if deposited, are ignored.