If like me, you like wandering around sand dunes – perhaps whilst you are on holiday by the seaside – keep an eye out for sand wasps. In particular, digger wasps like this which make nests in the sand for their young to develop in. The species shown here (Bembix rostrata) is only found in the Channel Islands in Britain, but there are many other digger wasps in the UK, with interesting biology like this one.
The Bembicini are a group (taxonomically, a Tribe) of predatory wasps which includes a genus of fairly large, brightly colored insects called Bembix sand wasps. There are about 20 species in Europe – see here – especially in the southerly Mediterranean countries such as Spain (where there are at least 10 species) and Italy. One of the most common and widespread of these is Bembix rostrata, which can be found as far north as Denmark and Sweden. I came across this species in Galicia in NW Spain, where these photographs were taken.
The nests of Bembix wasps consist of a tunnel – of varying length, depending on the texture of the soil – which ends in one or more cells where the female lays an egg and places a prey item for her offspring to feed on. Bembix wasps mainly capture flies (dipterans); they are not too fussy about what sort of flies they catch, probably whatever is available, but a study of another species found (only) in Spain – Bembix merceti – showed that nearly 90% of flies were bombylids, or bee flies (like the one shown below). Bee flies are pretty spectacular fliers themselves, fast and agile, so it demonstrates just how formidable a predator a sand wasp is!
In sandy habitats during the summer, it is common to come across these busy little (15–24 mm) wasps, flying close to the ground – looking for a good nesting site, or actively digging a nest tunnel. The sand grains fly out behind them as they dig away in the sand (below). It usually takes the females several days to make a nest and it is said that they mainly do this in the afternoon, presumably when they are nice and warm in the sunshine.
The sand dunes at Morouzos Beach (playa de Morouzos in Spanish) where I came across these wasps at the end of June last year (2017), lie at the entrance to the Rias Ortigueira and Ladrido in A Coruña Province, Galicia, Spain are Special Protection Area (SPA) – under the EU Directive on the Conservation of Wild Birds – known as ZEPA in Spanish. Despite being quite a delicate and dry habitat, there are an abundance of flowers, particularly the yellow Helichrysum flowers which are favoured by sand wasps as a source of nectar (see top photo).
Sand wasps such as B. rostrata frequently nest in large colonies – consisting of up to several hundred insects – where the females cluster together to construct their nest tubes – up to 20 cm long – each containing just one brood cell. Aggregations may include sand wasps of more than one species. There is at least one other species nesting in these dunes, Bembix oculata, which I came across feeding on a flower (below). B. oculata is another species which often nests in colonial aggregations in sandy soils; its burrows can reportedly extend up to 40 cm in length, sloping down to a depth of 17 cm deep, depending on the consistency of the soil. I can only imagine that these nests must require a great deal of maintenance in such dry, sandy soil.
There comes a time for mating. The males are usually the first to appear early on in the season, and they patrol over the nesting area looking for emerging females. Sometimes they even dig furiously to search for females as they emerge from the soil! When a a female appears, the males immediately try to copulate with her according to Schöne & Tengö (1981): “Often one male (catches the female and) carries her away in flight, followed by a trail of other males”. After this, which seems more or an abduction than a courtship, the females get on with the work of creating the next generation: alternately digging burrows and feeding on flowers. Apparently, patrolling males can interfere again: “pouncing upon the digging or feeding female. If she flies away a male follows, often joined by other males. Sometimes the female is pounced upon when in flight. The couple falls down, the other males follow, and a ball of males might be formed around the rejecting female” (Schöne & Tengö, 1981). About a week after she has emerged, the female starts bringing the first flies to the nest.
It take about two weeks for the developing lava to become an adult; during which time the caring mother will have provided numerous flies for it to feed on; she carefully reseals the nest after each visit. Quite what the males do during all this time, apart from harassing the females (who can only mate once in any case) I have no idea!
Despite the care which is taken to keep the nests sealed and protected, there are many insects which exploit the developing sand wasps for their own offspring. These parasites include of the genus Parnopes (Chrysidinae), including Parnopes grandior – a so-called cuckoo wasp – which specialises in B. rostrata and B. oculata.(see below). Somehow, the female P. grandior cuckoo wasps can dig up the nests of their hosts and close them again after they have laid their own egg. Apparently, they do this very quickly, which would be fascinating to see, although they are very small (8-13mm), so it would be extremely difficult to photograph in the wild. The host larva (i.e. of the sand wasp) is not killed until it is fully developed. I suppose it is more efficient to wait until the mother sand wasp has fattened up her developing larva, before killing and eating it! How macabre, but that’s Nature folks! And without the cuckoo wasps, I guess the sand dunes would be heaving with sand wasps. I did once come across a tiny cuckoo wasp like this one but did not manage to get a photograph!
Hunting wasps, including this species, were famously studied and described by Jean-Henri Casimir Fabre (1823 – 1915), but perhaps that is best left to another blog.
Asís, J. D., Tormos, J., & Gayubo, S. F. (2004). Nesting behaviour and provisioning in Bembix merceti and Bembix zonata (Hymenoptera: Crabronidae). Journal of Natural History, 38(14), 1799-1809.
Ballesteros, Y., Tormos, J., Gayubo, S. F., & Asís, J. D. (2012). Notes on the prey, nesting behaviour and natural enemies of three Bembix sand wasps (Hymenoptera: Crabronidae) in the Iberian Peninsula. In Annales de la Société Entomologique de France (Vol. 48, No. 3-4, pp. 281-288). Taylor & Francis Group.
Ballesteros, Y., Polidori, C., Tormos, J., Baños-Picón, L., & Asís, J. D. (2014). Complex-to-predict generational shift between nested and clustered organization of individual prey networks in digger wasps. PloS one, 9(7), e102325.
Paukkunen, J., Berg, A., Soon, V., Ødegaard, F., & Rosa, P. (2015). An illustrated key to the cuckoo wasps (Hymenoptera, Chrysididae) of the Nordic and Baltic countries, with description of a new species. ZooKeys, (548), 1.
Polidori, C., Ballesteros, Y., Santoro, D., Tormos, J., & Asís, J. D. (2012). Morphological distance and inter-nest distance account for intra-specific prey overlap in digger wasps (Hymenoptera: Crabronidae). Population ecology, 54(3), 443-454.
Schöne, H., & Tengö, J. (1981). Competition of males, courtship behaviour and chemical communication in the digger wasp Bembix rostrata (Hymenoptera, Sphecidae). Behaviour, 77(1), 44-65.