Volatile Organic Compounds Release Under Threat in Bumblebees: Chemical Identification and Antennal Detection.

Alarm behaviors have been widely studied in social insects such as termites, ants, honeybees and stingless bees. As part of this behavior, it is well known that alarm pheromones play a crucial role in recruiting nestmates to defend the colony. Despite the acknowledged phylogenetic proximity between bumblebees, honeybees and stingless bees, there is no evidence indicating the presence of alarm pheromones in bumblebees of the genus Bombus. Herein, during experiments involving Bombus pauloensis foragers under perceived threat, we detected a strong odor, prompting us to investigate the composition of the released secretion. By means of Solid Phase Microextraction and Gas Chromatography-Mass Spectrometry, we found more than 20 volatile compounds, with 2-Heptanone and both enantiomers of Rose oxide being more predominant. These volatiles were quantified. In addition, to determine the secretion source, we evaluated the antennal response to odors coming from different dissected body parts (e.g., head, mandibles, sting) of foragers, and found that the responses were significantly different to odors coming from the dissected mandibles. Our results provide the first chemical description of a particular repulsive blend released by bumblebee foragers when threatened. In this study, we leverage our understanding of B. pauloensis chemical communication in order to glean new insights into potential shared and elaborated mechanisms mediating social organization across bumblebee species.