Identity and function of scent marks deposited by foraging bumblebees

Foraging bumblebees can detect scents left on flowers by previous bumblebee visitors and hence avoid flowers that have been depleted of nectar. Tarsal secretions are probably responsible for this repellent effect. The chemica l components of the tarsal glands were analyzed by combined gas chromatogra phy-mass spectrometry for three species of bumblebee. Bombus terrestris, B. lapidarius, and B. pascuorum. The hydrocarbons identified were similar for each species. although there were interspecific differences in the relativ e amounts of each compound present. The tarsal extracts of all three specie s comprised complex mixtures of long-chain alkanes and alkenes with between 21 and 29 carbon atoms. When B. terrestris tarsal extracts were applied to flowers and offered to foraging bumblebees of the three species. each exhi bited a similar response; concentrated solutions produced a repellent effec t, which decreased as the concentration declined. We bioassayed synthetic t ricosane (one of the compounds found in the tarsal extracts) at a range of doses to determine whether it gave a similar response. Doses greater than o r equal to 10(-12) ng/flower resulted in rejection by foraging B. lapidariu s. Only when less than or equal to 10(-14) ng was applied did the repellent effect fade. We bioassayed four other synthetic compounds found in tarsal extracts and a mixture of all five compounds to determine which were import ant in inducing a repellent effect in B. lapidarius workers. All induced re pellency but the strength of the response varied: heneicosane was most repe llent while tricosene was least repellent. These findings are discussed in relation to previous studies that found that tarsal scent marks were attrac tive rather than repellent.