Yeast volatiles promote larceny in bumble bee behavior

The proximate mechanisms that encourage flower visitors to choose to seek benefits without conferring a service in return, such as robbing flowers of their nectar without providing any pollination, remain largely unknown. In this study, we combined field and laboratory assays with gas chromatography-mass spectrometry (GC-MS) to test the hypothesis that a nectar-inhabiting yeast can mediate exploitative behaviors displayed by bumble bees with flexible foraging tactics. In field assays, Bombus bifarius and Bombus flavifrons secondary nectar robbers discovered robbing holes 2.2 s faster on average and foraged 63% more on Metschnikowia reukaufii-inoculated flowers of both Corydalis caseana and Mertensia ciliata than on controls (or uninoculated flowers). Olfactometer assays confirmed a yeast volatile-mediated response, with nectar scent enhanced with 14 volatile compounds when fermented with yeast for 2 days relative to unfermented nectar. Collectively, our results suggest an important role for yeast odors in mediating exploitative behaviors displayed by mutualist species, acting as a potential reliable cue for resource presence and improving bee foraging efficiency.