Striking in appearance - they are almost the equivalent of human supermodels - and readily seen, Ammophila digger wasps are one of the best studied groups of solitary wasps in the world, initially through the brilliant research of Gerard Baerends in the Netherlands and subsequently by such experts as Professor Jeremy Field in Britain. One bonus for researchers is that the behaviour of the genus is varied and fascinating.
The two species in Britain are the relatively common Ammophila sabulosa (up to 25mm) and the nationally scarce Ammophila pubescens (up to 19mm), which is distinguished by its smaller size, lack of a blue tinge towards the tip of the abdomen and - by far the best guide - slightly different markings on the forewings.
Both species are seen at the height of summer and nest in level soil, principally in heathland or coastal sand dunes, where they sometimes share the habitat. Burrows are single-celled and shallow, only a few centimetres deep, and prey consists of a wide variety of moth caterpillars occasionally as much as ten times the weight of the wasp.
The nests are mass provisioned by Ammophila sabulosa, which deals with one nest at a time to a maximum of ten, and provisioned progressively by Ammophila pubescens, which can manage up to three nests simultaneously.
Mass provisioning involves catching as many caterpillars as the individual grub will need, between one and five, then sealing the burrow. Progressive provisioning consists of catching a small number of prey, perhaps two, then closing the burrow temporarily. The female then returns to check on her progeny's progress and, when required, catches and inserts additional caterpillars in the nest up to a final total of ten.
Clearly with this method used Ammophila pubescens needs not only considerable skill at foraging but also at locating her respective nests, which can all be at a different stage of development. To this end, like some other solitary wasps, she uses nearby 'landmarks' to identify where the separate burrows are. Suitable objects include stones, pieces of wood and plants.
The female Ammophila sabulosa pictured above with a pug moth caterpillar (Eupithecia/Gymnoscelis sp.) was probably concluding the provisioning of this nest. After the initial stage of leaving a couple of caterpillars this species does not cover her burrow with the full vigour that follows later. (Prey is always pulled in backwards after being transported on foot or sometimes in flight with small examples.)
With additional prey, more effort is made to fill in the burrow, which is then covered fully with sand applied by rapid movement of the legs. However, this takes only a minute or so while the final closure takes significantly longer. This is done with carefully chosen stones, twigs or lumps of soil that are finally covered by sand, a process which, according to Baerends, invariably involves tamping down with the head as the wasp goes almost vertical. I haven't seen this with any of the Ammophila pubescens in Surrey. The development of the larva is rapid, taking just ten days.
Perhaps the most interesting element of the nesting behaviour of the two species is parasitism, either through replacing the host's egg in a cell without taking the caterpillar(s) away (brood parasitism), or removing prey from inside a nest and using the booty to provision the thief's nest.
This is done to an astonishing degree - for Ammophila sabulosa, Professor Field noted that of 32 marked females active for at least 40 hours of observation, 24 opened at least one other female's nest.
Ammophila will also try and seize caterpillars off each other, saving the effort of hunting. The most extreme example of this I have seen was by the female Ammophila pubescens pictured above at Thursley Common in Surrey, who managed to get hold of a temporarily abandoned Pine Beauty Moth (Panolis flammea) caterpillar and attempted to get it to her burrow. As can be seen from the image, the caterpillar was vastly bigger than the wasp who eventually had to give up her efforts.
Ammophila are also distinctive from the majority of solitary wasps in being able to take nectar from a fairly wide variety of flowers. Whereas most have short tongues, forcing them to use principally umbellifers headed by Wild Carrot and Wild Parsnip, Ammophila's longer tongues enable them to utilise such plants as Ling and Rosebay Willow-herb as well as Thistles in the image above.
Images © Jeremy Early. All rights reserved.
In 2013 I published My Side of the Fence - the Natural History of a Surrey Garden. Details may be found, and orders placed, via this hyperlink My Side of the Fence. In November 2015 Surrey Wildlife Trust published the atlas Soldierflies, their allies and Conopidae of Surrey, jointly written by David Baldock and me. Details are on this web page: Atlas.