Crabronidae: Square-headed wasps, Sand wasps

Biology

The Crabronidae is one of our most diverse wasp families, with over 1,200 species in the U.S. and almost 9,000 species worldwide.  Most species are also unique in terms of their food choices, nesting locations and behavior.  Adult crabronid wasps derive all of their nutrition by visiting flowers, and they are often seen in late summer on flowers such as milkweed (Asclepias spp.) and goldenrod (Solidago spp.).  With a few exceptions, most crabronid wasps are predatory.  They paralyze prey insects including aphids, leafhoppers, flies, beetles, crickets, cicadas, spiders and caterpillars and fly them back to a nest in the ground, a plant stem or a rotten log.  One of the largest crabronids in the Pacific Northwest is the two inch long brown and yellow-banded Pacific Cicada Killer (Sphecius convallis).  The adults appear in June and July and are seen visiting a wide range of flowers.  The females are sometimes seen digging their ten-inch-long tubular nests in sandy soil.  Oval chambers are excavated at the end of the burrow, large enough to accommodate a few cicadas. Cicadas are usually captured in flight, paralyzed by venom but remain alive while being fed upon by the wasp larva for two weeks before pupation.

Sand wasps comprise a subfamily of Crabronidae and are familiar wasps in sandy or light soil landscapes where they make their nests.  Sand wasps are predators on many kinds of insects but most are fly predators.  It is common for numerous females to excavate nests in a small area, creating large and sometimes dense aggregations.  Most sand wasps are yellow and black although some are black and white with green eyes like the common American Sand Wasp (Bembix americana).

This species is a medium sized (about ½ inch long) wasp with an unmarked but hirsute grey thorax, blue grey markings on the abdomen, and mostly yellow legs.  Adults feed on nectar and pollen with the latter transported effectively thanks to the hairy thorax.  Females progressively provision their nests as opposed to providing a finite food source for the larva and then moving on.  Sand wasp females continue bringing flies to the larval wasp until development is complete, in the manner of a bird feeding its developing chicks.  After the larva has finished feeding, the female seals the nest, and the larva creates a cocoon and overwinters in it as a pre-pupa.

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