During June this year I made a special effort to observe the behaviour of the ‘chimney’ potter wasp Odynerus spinipes. This fascinating wasp nests in burrows on vertical clay cliffs and constructs downward curving mud chimneys over the nest entrances. The wasp is mainly found on the coast so I returned to a spot at Prawle where I had previously watched these wasps during 2014.
During a visit on 26 May two newly emerged potter wasps were seen around the low clay cliff where they nest but neither stayed for long. The beautiful cuckoo wasp Chrysis viridula is a specialist parasitoid of the Odynerus and several were flying around the cliff that day.
After a week away I returned on 5 June hoping to see a wasp construct her burrow. I was in luck. At 11.40 am a female potter wasp was inspecting the cliff face and after a short time she visited a nearby puddle to collect water. Returning to the cliff she wetted an area of clay and started digging out pellets of mud. These were applied to the edges of the hole and formed the base of the chimney. After a short while she gave up probably after reaching an obstruction in the clay. She started another burrow nearby but again gave up. Then at 12.10pm she started a third burrow and this was successful. For the next hour and a half the wasp returned to the burrow with fresh water supplies at about 5 minute intervals and rapidly dug out pellets until just her abdomen showed above ground. I was surprised at how quickly she worked building up solid rings of mud for the base of the chimney. The chimney was about a centimetre long when she finished. This was not as long as others I had seen but must have resulted from her excavating just the first nest chamber. These wasps are known to construct 5 or 6 nest chambers in each burrow.
The wasp then flew off and returned with two small green weevil larvae at 2.18 and 2.48 pm. The purpose of the chimney is currently unknown, there are various theories about them ranging from the chimney protecting the burrow from rain to it being some kind of defence against nest parasites such as the cuckoo wasp and various flies or as a form of nest temperature regulation.
The chimneys are very delicate and easily broken. In fact whilst watching the wasp dig her burrow I accidentally broke the chimney over a nearby burrow. This wasp was bringing in weevil larvae and didn’t seem that bothered that the chimney had suddenly disappeared. Shortly after this the wasp must have completed stocking her nest cell (they are known to use about 25 weevil larvae to do this. One egg is placed in each cell and the potter wasp larva emerges after a few days and consumes the paralysed weevil larvae before spinning a cocoon in which to overwinter. The larva will pupate in the spring with the new wasp emerging in late May). She then began to dig out a new nest chamber below ground and quickly constructed a new chimney.
From this and observations made on several other visits I learnt that the longer chimneys were the result of the wasps digging out more chambers underground. The longest chimneys were about 3 cm long. Also when watching wasps digging out new chambers I noticed that they either brought out small pellets which were applied to the chimney base in short interwoven lines which formed a latticework of mud rather than a solid structure. During this process the wasp would also dig out larger mud pellets which were discarded from the end of the chimney.
Often when I returned to the site after a few days of wet weather I found that almost all the chimneys had been washed away by heavy rain. This discounts the rain theory and I think it most likely that these structures play a role in regulating the temperature of the nest. During hot weather the south-facing cliff where they choose to nest bakes in the sun so it seems likely that at these times the chimneys somehow regulate the air flow around the nest and cool its contents.
When the female potter wasp has completed her set of about 6 cells in the burrow she seals the nest entrance. To do this she returns to the puddle and collects water. Copious amounts of water wet the nest entrance and then mud from the chimney, if it is still there, and from the surrounding area is used to quickly seal it up. The female will then start searching for a new site and start a new burrow. This may be close to the first and several burrows are often found close together. At this site there were nest burrows constructed along about 50 metres of cliff. The reason they are all close together is probably because there are permanent puddles here on the flat rocks at the top of the beach. The only other spot I have found these wasps nesting in the area was where water seeped through the cliff on to the beach.
Whilst watching the potter wasps there were always several cuckoo wasps Chrysis viridula around the nest site. These small, beautiful metallic green and pink wasps were hyperactive during hot weather. Early in the season most were males waiting for females to emerge from last years nests in the cliff face. Occasionally a male would be seen mounting a female and mating with her, this lasted about 5 seconds. Towards the end of June and in July just the females remained and were constantly inspecting the nest burrows often disappearing inside for a few minutes. The female cuckoo wasp seeks out a chamber with a full grown potter wasp larva in its cocoon. After breaking and entering she lays her egg beside the potter wasp larva, the emerging cuckoo wasp larva then sucks the insides out of the unfortunate potter wasp larva before spinning a cocoon to overwinter in. The next generation of cuckoo wasps then emerges with the potter wasps the following spring.
During my last visit on 13 July I watched a female constructing a new burrow and these wasps are known to fly in to August but I felt it was time to move on and find some other insects to study. By this time I had spent over 30 hours observing these wasps and had learnt a great deal about their ecology. I must have looked a strange sight to other visitors to the beach as I stood for hours on end seemingly staring at a bare cliff face in the baking sunshine!