German wasp (Vespula germanica) (GFDL 1.2, https://commons.wikimedia.org/w/index.php?curid=1931193)

As I bent down over the wasp nest taking pictures with my mobile phone (below) their mood suddenly changed and I saw the wasps flying up towards me! I ran away as fast as I could, but they followed me for over 100 metres,  and I was lucky to get away with only four stings! Looking back, what was amazing was how quickly they became aggressive. What form of subtle communication changed them from a peaceful social insect into a horde of killers?!

German wasp (Vespula germanica) nest entrance 21 Aug 2020. Photo by Ray Cannon

When I first came across this wasp nest on 23 July, it was nicely hidden in the grass of a meadow (below). On this occasion I had camera with a 150mm macro lens, which meant that I could stand back a bit and take photographs without disturbing them too much.

German wasp (Vespula germanica) nest entrance on 23 July 2020. Photo by Ray Cannon

The wasps were coming and going from the nest; other were repairing the entrance, their mandibles full of mud (below), but they did not appear to be overly upset by my presence.

German wasp (Vespula germanica) at nest entrance on 23 July 2020. Photo by Ray Cannon

On my second visit, about a month later, the nest was much more exposed (below); perhaps dug out by a badger, someone suggested. Nevertheless, the wasps were still at work and reasonably peaceful as I started to take some pictures. However, on this occasion I only had my mobile phone with me, so I had to hold it much closer above the nest entrance than I had done so before. They put up with it for a while, but then they snapped!

German wasp (Vespula germanica) nest entrance on 21 Aug 2020. Photo by Ray Cannon

To understand the nature of their sudden change in behaviour, I did some research. I knew about alarm pheromone in aphids – see video here – which causes them to stop feeding an walk away or drop off the plant. Alarm pheromones also play a key role in social insects, but their function in social wasps and hornets still remains relatively poorly understood compared with other social insects.

German wasp (Vespula germanica) face by nutmeg66 (Flickr CC) (https://www.flickr.com/photos/rachel_s/5991018152/) Creative Commons

In bees and  wasps, alarm pheromone is a call to action; a chemical signal which rouses a colony to defend itself. It also marks the intruders for attack! Several social wasp alarm pheromones have been discovered, and most of these are in the venom sacs of the wasps.

German wasps (Vespula germanica) landing on an apple (https://www.flickr.com/photos/fotoopa_hs/3778681208/) Creative Commons

In both German (Vespa germanica) and Common wasps (V. vulgaris) alarm behaviour was found to occur in response to a squashed sting apparatus, or a sting sac, or the solvent extract of the sting sac (Maschwitz 1964). The venom is the source of alarm pheromones (Bruschini et al., 2008)

German wasps (Vespula germanica) emerging from nest by nutmeg66 (https://www.flickr.com/photos/rachel_s/5991018136/) Creative Commons

In social wasps like these, the nest guards – they were the ones around the entrance I suppose – can release alarm pheromones to recruit other wasps to help defend the nest. Sting venom is the primary source of alarm pheromone in many different species of wasp – and is released when sting venom is exuded by the stinger, or via stinging – but in some species (Vespula spp.), mandibular glands may also provide alarm pheromones (Reed and Landolt, 2000). So it is not altogether certain whether the chemical is coming just from the venom, or from the mandibles as well. Indeed, it may not just one chemical acting alone, but rather several molecules acting in concert that triggers the alarm behaviour: see here. Scope for some more research.

German wasp (Vespula germanica) Queen in hibernation by MaxNikon / CC BY (https://creativecommons.org/licenses/by/4.0) Creative Commons

Finally, it is interesting to note that the Asian hornet (Vespa velutina)(below) also uses sting venom volatiles as an alarm pheromone (Cheng et al., 2017). The alarm pheromone is produced by the sting gland, and increasing quantities of sting gland extract increased aggressive attacks. In my experience, see blogs here and here, these invasive hornets are remarkable peaceful if you do not upset them, but it may be a different story when one approaches their nest. Only trained pest control experts should approach their nests and remove them. N.B. the Asian hornet is not established in the UK, but is now widespread on the continent of Europe.

Asian hornet (Vespa velutina) from Spain. Photo by Ray Cannon

References

Cheng, Y. N., Wen, P., Dong, S. H., Tan, K., & Nieh, J. C. (2017). Poison and alarm: the Asian hornet Vespa velutina uses sting venom volatiles as an alarm pheromone. Journal of experimental biology, 220(4), 645-651.

Maschwitz, U. W. (1964). Alarm substances and alarm behaviour in social Hymenoptera. Nature, 204(4956), 324-327.

Reed, H. C., & Landolt, P. J. (2000). Application of alarm pheromone to targets by southern yellowjackets (Hymenoptera: Vespidae). Florida Entomologist, 193-193.