I had the good fortune of witnessing an industrious female sand wasp excavating her nest at Lathrop State Park in Huerfano County, Colorado last August 4, 2014. Insect activity was minimal in the late afternoon as the wind was picking up a bit and a thunderstorm was quickly approaching. While my wife was looking for a geocache, I studied the nearby vicinity and noticed a hole in the sand. Before I could complete the thought of "Hm-m-m, I wonder what dug that?," a sand wasp backed out of the cavity.
The industriousness of a Bembix female is something to behold, as you can see in the video below. She rapidly kicks out large quantities of sand using a "tarsal rake" of spines on each front leg.
There are twenty-one species of Bembix in North America north of Mexico, so generalizations about their behavior are risky. Still, the burrows are oblique, nearly horizontal in many cases, ranging from 19-57 centimeters in length, and a depth of 5-28 centimeters. The tunnels may curve, and usually end in a terminal cell. Several species also dig short, dead-end burrows or furrows in the immediate vicinity, probably to confuse parasites. The entrance of the real, finished burrow is thoroughly concealed.
The burrow is excavated before the wasp goes hunting. True flies in the order Diptera, exclusive of the suborder Nematocera, are used as prey. Bembix are generalist, opportunistic hunters. A victim is paralyzed or killed by the wasp's sting, and is then flown back to the nest. She uses subtle landmarks to unerringly find the buried entrance. Meanwhile, we can't remember where we parked our car. Most of our common Bembix species will lay an egg on this first victim, but some species lay an egg in the empty cell before commencing the hunt.
Once the egg hatches, the mother wasp brings flies to her larva as needed. This is called "progressive provisioning" and is more typical of parental care in birds or mammals than in insects. When the larva reaches maturity, the female wasp closes the cell. Inside, the larva spins an oblong cocoon, weaving sand grains into the structure and resulting in a hardened capsule. Overwintering takes place as a prepupa inside this cocoon, but there are usually two generations annually.
The mother sand wasp may fill in her burrow once her single larva reaches maturity, or she may construct one or two additional cells, each at the end of a short tunnel branching from the main burrow.
Male sand wasps are often seen alighting on the ground amid the numerous burrows of females, but they also participate in elaborate flight rituals called "sun dances." Males emerge before females, and fly at erratically at dizzying speed one or two inches above the ground attempting to detect virgin females about to erupt from their underground chambers.
Females join the males in flight if they are not pounced on immediately, and a pair that unites in mid-air will make a bee-line out of the mob and finish mating elsewhere before re-joining the masses. Should a pair tumble to the earth, great numbers of males will try and usurp the initial suitor.
Both sexes fuel their frenetic lifestyle with flower nectar, especially from composites (flowers in the aster family). Some of their mouthparts are fused into a tongue-like proboscis they use to probe for nectar. Sand wasps do nothing slowly it would appear, and one barely gets a glimpse of them, even at a flower, before the wasp is off to another destination.
Bembix wasps are plagued by the usual suspects that parasitize wasp nests: Cuckoo wasps (Chrysididae), velvet ants (wasps in the family Mutillidae), satellite flies (Sarcophagidae), and bee flies (Bombyliidae) being the chief villains in the sand wasp world. Additionally, the adult wasps can be victimized by parasitic thick-headed flies (Conopidae), or killed outright by robber flies (Asilidae).
Watching a nesting aggregation of sand wasps is never a disappointment, and at the very least you will find joy in each and every pesky fly they dispatch in providing for their larval offspring.
Sources: Bohart, R.M. and A.S. Menke. 1976. Sphecid Wasps of the World. Berkeley: University of California Press. 695 pp.
Evans, Howard E. 1966. The Comparative Ethology and Evolution of the Sand Wasps. Cambridge, MA: Harvard University Press. 326 pp.
Rau, Phil and Nellie. 1918. Wasp Studies Afield. Princeton, NJ: Princeton University Press. 372 pp. (Dover edition, 1970).