The chemistry of an unusual seed dispersal mutualism: Bees use a complex set of olfactory cues to find their partner

Many insect groups are important mutualistic partners of plants. Bees in particular provide an essential mutualistic service to plants: pollination of their flowers. They can also act as seed dispersers for plants, a rare seed dispersal mutualism termed melittochory. One group of known bee seed dispersers are stingless bees (Apidae: Meliponini). Australian stingless bees collect resin from fruits of the eucalypt tree Corymbia torelliana (Myrtaceae). In doing so, they disperse the trees' seeds. Therefore, the resin chemistry of C. torelliana may be particularly well adapted to attract the bees. While the importance of visual and olfactory cues has been frequently studied in pollination mutualisms, we here provide the first study to investigate the importance of resin compounds as olfactory cues in a bee seed dispersal mutualism. To investigate whether the seed-dispersing Australian stingless bee species Tetragonula carbonaria relies on olfactory cues when searching for C. torelliana resin and which compounds of the blend they use to identify it, we performed several choice tests with pure resin extracts, fractionated extracts, extracts enriched with C. torelliana resin compounds and with single compounds. Olfactory cues were sufficient to attract bees, confirming the importance of olfactory cues for resin-searching stingless bees. However, bees only landed on filter papers with pure resin extract, but not on fractionated or enriched extracts, nor on single compounds. This finding indicates that, instead of relying on just a few specific volatile compounds, the bees used a complex blend as recognition cue including relative compound proportions. Moreover, they showed little tolerance for even slight concentration changes of single components. Such high cue specificity is rather unusual for insects seeking resources, but does, in the case of Australian stingless bees, ensure detection and identification of the resin of their mutualistic partner.