Every wasp knows its place in society: queens lay eggs; workers toil. But what happens when a nest is under attack? Claudia Bruschini explains that as soon as a guard wasp stings an attacker, the venom releases an alarm-signalling odour picked up by the workers before they swarm to the colony's defence. But queens can't rely on protection from their workers in the days soon after establishing a nest. Only when her first eggs have hatched at the end of May can the queen turn to her nest mates for help. Which made Bruschini and her colleagues, Rita Cervo, Ilaria Protti and Stefano Turillazzi from the Università degli Studi di Firenze, Italy, wonder whether the queen's sting could convey the same wasp-rousing messages that workers' stings send (p. 2442).
First they had to choose a species of social wasp to answer the question for them. Bruschini explains that paper wasps (Polistes dominulus)are ideal; they construct simple open nest structures, which are perfect for studying the insects' comings and goings. Having settled on their wasp of choice, Bruschini and her colleagues headed into the Tuscan hills where the wasps like to build their nests in old tombs. Mounting an early morning raid on a cemetery in a near-by mountain village, the team collected wasp nests before the day warmed and the wasps became active.
Back in the lab, the team measured the size of worker and queen wasp venom sacs, and found that they were essentially the same size. But when they compared the venom sac sizes relative to the insects' body sizes, `the workers' venom reservoirs were much bigger,' says Bruschini. The workers invest significantly more in venom production than the egg-laying queens.
Next the team analysed the alarm signalling volatile compounds in the queens' and workers' stings. `This took a long time,' Bruschini admits. Carefully collecting minute drops of venom from individual venom sacs, the team analysed the volatile compounds with mass spectrometry and successfully identified 20 volatile components of the more than 40 found in both venoms. The team could see that the stings were similar: they shared many of the same compounds, but there were higher levels of spiroacetyls in the worker venom and an acetamide in the queens'. How would the wasps react to both stings?
Bruschini, Cervo and Protti headed back into the countryside to a field where they had set up attractive shelters for the queens to nest in. Knowing that the wasps only reacted to alarm signals coupled with a visual stimulus,the team rigged up a black neoprene disc in front of the nest for the wasps to focus their attack on. Then they set the disc oscillating, released a drop of venom behind the nest and waited to see how the insects responded.
As soon as the queen's venom was released, the workers mobilised but limited their aggression to flying towards the bobbing disc. However, it was a different matter when the workers' venom was released. The workers mounted a full-scale attack, landing on the disc and even stinging it. While queen's venom was sufficient to stimulate the early stages of an attack sequence, only the volatile components of the workers' venom provoked a full-on assault.
Despite their similarities, the two venoms are sending different messages,and Bruschini is keen to find out which venom components mobilise an attack.