Wasps in the Sand

Sand wasp Bembix oculata with prey (a bombyliid, I think), copyright Carlos Enrique Hermosilla.

The sand wasps of the tribe Bembicini are a diverse group of about 500 species of wasp that get their name because, obviously, of their habit of constructing burrows in sand-banks. Like other members of the wasp family Crabronidae, sand wasps provision these burrows with food for their larvae in the form of other insects. The adults themselves feed on nectar. Some of the Bembicini are among the larger members of the Crabronidae, and most of them are strikingly marked in black and yellow, white or red. Other characteristics of the group include an elongate labrum above the mouth, and the reduction of the ocelli, often to simple scars.

Bembicins are divided between a reasonable number of genera (Bohart & Menke, 1976, listed fifteen, but subsequent authors have recognised more) but the greater number of species are included in just one of these, the cosmopolitan Bembix with over 300 species (some older sources spell this name 'Bembex', but Bembix seems to be correct). Bembix is also the only genus found outside the Americas. Bohart & Menke (1976) suggested three main lineages within the Bembicini: one containing the relatively plesiomorphic genera Microbembex and Bicyrtes, a group of four genera including Stictiella and Glenostictia in which the ocelli are sunken into pits, and a large group containing genera related to Bembix with a raised welt at the front of the scutum on the thorax.

Sand Wasp - Bembix americana from Dick Walton on Vimeo.

While other wasps will lay their egg(s) on a paralysed insect in a brood chamber and then fly off never to return, many bembicins continue to bring fresh food items to the burrow throughout their larvae's development (take a look at the video above, by Dick Walton). The majority of bembecins provide their larvae with flies as food, which they paralyse with their sting and then carry back to the burrow between their mid-legs. Only a small number of genera regularly use other prey, though notably these genera include both Bicyrtes and Microbembex (so predation on flies is possibly ancestral for a clade excluding these two genera rather than for the tribe as a whole). Bicyrtes species stock their burrows with bugs (Heteroptera), most commonly nymphs. Microbembex species are the gourmands of the tribe, taking prey ranging from mayflies to midges. They are also somewhat unusual in that they stock their burrows with dead as well as paralysed insects (because they are providing food continuously, freshness over an extended period is less important than it is for other wasps). Females may compete for dead insects: in the words of J. Parker (as quoted by Bohart & Menke, 1976): "The struggles at the mouth of the burrow for the possession of a dead insect are frequent and furious, the contestants grappling and rolling over and over on the sand. Frequently it happens that the prey is dropped in the struggle, and while the pair of contestants are rolling on the sand a third wasp comes along and settles the quarrel by quietly carrying off the coveted treasure". Of the other bembecin genera, species of the genera Stictiella and Editha are predators of Lepidoptera. Editha species are found in southern South America, and include the largest of the bembecins. Xerostictia longilabris, a member of the Stictiella-group from southern North America that gets its own genus, has been recorded stocking its burrow with ant-lions and flatid bugs (Evans 2002). Members of other genera may also stock their burrows with prey other than flies, though in the majority of cases they do not do so exclusively (Evans 2002).

Sand wasp, possibly Microbembex monodonta, burying the entrance to her burrow. Copyright Tim Lethbridge.

Many bembecins (most notably in the genera Microbembex and Bembix) nest gregariously, and may form sizable colonies. Species of Bembix will maintain the same colony from year to year. In some species, such as B. pallidipicta, the females may dig accessory burrows near the main burrow without laying eggs in them; these have been presumed to act as decoys to discourage parasitoids or kleptoparasites. Males perform prolonged 'sun dances' above the colony in which they fly in circles or figures-of-eight, looking out for attractive females (O'Neill 2001). Males of many species have a serrated mid-femur that they use to hold down the female's wings while mating; in species without these leg serrations, mating may involve more of a struggle (Bohart & Menke 1976). The male produces a loud chirping during mating, presumably just to add atmosphere.

REFERENCES

Bohart, R. M., & A. S. Menke. 1976. Sphecid Wasps of the World: a generic revision. University of California Press.

Evans, H. E. 2002. A review of prey choice in bembicine sand wasps (Hymenoptera: Sphecidae). Neotropical Entomology 31 (1): 1-11.

O'Neill, K. M. 2001. Solitary Wasps: behavior and natural history. Cornell University Press.