Orchid Fragrance Complexity as a Mechanism for Euglossine Bee Pollinator Specialization

Neotropical orchids of subtribes Stanhopeinae and Catasetinae are exclusively pollinated by euglossine bees (Euglossini). This study determines whether orchids present specific combinations and proportions of chemical odors in order to attract a limited subset of euglossine bee species, thereby avoiding hybridization that may result from the attraction of too many pollinators. Chemical baits of cineole, benzyl acetate, methyl salicylate, cineole/benzyl acetate (a Stanhopea tricornis fragrance analogue), and cineole/methyl salicylate/methyl benzoate (a Catasetum gnomus fragrance analogue) were used to attract fifteen species of euglossine bees in the San Luis Valley, Monteverde, Costa Rica. The diversity of bee species attracted to the cineole/benzyl acetate solution was significantly lower than either cineole or benzyl acetate (Shannon-Weiner index; for cineole/benzyl acetate: H’ = 0.89; for cineole: H’ = 1.91; for benzyl acetate: H’ = 1.82; for cineole/benzyl acetate and cineole: F = 4.89, df = 68.00; for cineole/benzyl acetate and benzyl acetate: F = 5.66, df = 64.29, p < 0.05). The diversity of species attracted to the cineole/methyl salicylate/methyl benzoate solution was significantly lower than cineole (ShannonWeiner index; for cineole/methyl salicylate/methyl benzoate: H’ = 0.82; for cineole: H’ = 1.91; F = 6.16, df = 61.25, p < 0.05). The results provide evidence that orchid fragrance complexes are designed to attract specific euglossine species that are both effective pollinators and specialized enough to reduce risk of interspecific outcorossing.