Alarm pheromones play a key role in social insects, but their function in social wasps and hornets remains poorly understood as compared to other social insects like bees and ants.

Vespa velutina  is an economically important and widespread hornet predator that attacks humans and honey bees. This species is native to Asia and has now invaded Europe. Despite growing interest in V. velutina, it was unknown if it possessed an alarm pheromone.

Prof. TAN Ken and his team of Xihsuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences conducted a study to identify the source of the alarm pheromone and determine its behavioral effects on hornet workers.

They chemically analyzed the pheromone and used electroantennography to identify active components and tested whether those components can elicit attacks. 

Their analyses of volatiles produced by alarmed hornets revealed that the alarm pheromone was produced by the sting gland, and increasing quantities of sting gland extract increased aggressive attacks. 

The researchers then used GC-MS analysis and authentic standards and identified 13 of the 16 major compounds found in those volatiles, all ketones. Using micro-scale derivatization, they narrowed the possibilities for two compounds (possibly a type of undecen-2-one and a type of undecene-2,10-dinone).  

They further found that worker hornets had antennal responses to four of the identified compounds (non-8-en-2-one, heptan-2-one, nonan-2-one, and undecan-2-one) and exhibited aggression to three of these compounds (heptan-2-one, nonan-2-one, and undecan-2-one). The alarm pheromone fractions (monoketones, diketones, and their combination) all significantly elevated attacks. 

The study provides the first evidence that V. velutina, a widespread and invasive species, uses an alarm pheromone. 

The study entitled “Poison and alarm: The Asian hornet Vespa velutina uses sting venom volatiles as alarm pheromone” has been published online in Journal of Experimental Biology.