Subtle Chemical Variations with Strong Ecological Significance: Stereoselective Responses of Male Orchid Bees to Stereoisomers of Carvone Epoxide

Different enantiomers of chiral compounds within floral perfumes usually trigger distinct responses in insects; however, this has frequently been neglected in studies investigating semiochemicals in plant-pollinator interactions. Approximately 1000 neotropical plants produce floral perfumes as the only reward for pollinators, i.e. male euglossine bees. The chiral compound carvone epoxide is a key component of the scent bouquet of many perfume-rewarding plants that are pollinated by males of Eulaema . Here, we tested the biological activity of the four carvone epoxide stereoisomers to four Eulaema species occurring in the Atlantic Rainforest of NE-Brazil. We determined the stereochemistry of carvone epoxide in the floral scent of several Catasetum species, tested whether the antennae of bees respond differentially to these stereoisomers and investigated if there is a behavioural preference for any of the stereoisomers. We found that 1) Catasetum species emit only the (−)- trans -stereoisomer of carvone epoxide, 2) for E. atleticana and E. niveofasciata antennal responses to the (−)- trans -carvone epoxide were significantly stronger than those to (−)- cis -carvone epoxide, 3) the strength and pattern of antennal responses to all 4 stereoisomers (separately tested) did not differ among Eulaema species, and 4) there were significant differences in attractiveness of the four stereoisomers to the bees species with the (−)- trans -stereoisomer being particularly attractive. We assume (−)- trans -carvone epoxide to be the dominant isomer in perfume-rewarding plants pollinated by Eulaema . The universal occurrence of carvone epoxide in Catasetum species pollinated by Eulaema , suggests that this compound has evolved in perfume-rewarding as a specific attractant for Eulaema bees as pollinators.