Once again, just last Saturday, July 28, I found myself baffled by a little wasp I found excavating a burrow in some sand not far from my home. I took images and, realizing that I didn’t recognize it, began the identification process. I started with something it reminded me of: a small Bicyrtes species. I then went up the classification ladder on Bugguide.net. How about the subtribe Bembecina? Not really, though Microbembex was close. Was it in the tribe Bembicini at least? Yes, that seemed like a safe bet. Browsing the images for the subtribes I found myself surprised that the Stizina looked promising. Indeed, it turns out I had been observing a female wasp in the genus Bembecinus.
The characters I found key to making the ID included the eyes, which are strongly convergent at the bottom of the face, and strongly divergent at the top of the head. The second submarginal cell in the front wing is petiolate or nearly so.
I thought I had lost my opportunity to photograph this wasp right off the bat. I made a quick move while she was down her burrow, but she popped back out quickly, saw me, and flew off. Luckily, she was determined to finish what she started, and returned shortly to continue digging. She would occasionally fly off, only to return a short time later, regardless of my presence or movements. At one point a tiny, curious ant walked into her burrow while she was inside it, and she literally kicked it out of her tunnel.
Good thing for the ant that female Bembecinus hunt leafhoppers (Cicadellidae and related families) as food for their larval offspring. Rather than stockpiling several prey and then leaving the larva to feed, Bembecinus practices an advanced form of parental care called “progressive provisioning.” She lays her egg in the empty cell at the end of her burrow, and then brings food to the larva on an as-needed basis. She can deliver, too. Researchers have recorded different species bringing in from 71 to 757 prey items to a single nest. One assumes the larger the prey species the fewer it takes to feed the wasp larva, but that is still a staggering amount of work for the mother wasp.
The burrow is eventually sealed permanently at, or before, the time the larva enters a pre-pupal stage. The female then begins the nesting process over again. Bembecinus is often highly gregarious, several to many individual females nesting in a small area. Indeed, I did find another specimen (though perhaps a different species) nesting nearby the first.
Interestingly, males of some species of Bembecinus actively dig to reach pre-emergent, virgin females. These males have short “tarsal rakes” of spines on their front feet, so may be mistaken for females themselves. They are among the most competitive of wasps when it comes to mating, and males may physically fight over a female at any point, including when another male is already coupled with a female.
Both genders gather in “sleeping clusters,” sizeable balls of wasps situated on twigs, stems, or foliage of plants near the nesting area. The clusters can be all males early in the season, and all females late in the season, though this varies with the species and maybe even the geographic area.
Bembecinus is not immune to nest parasites. Velvet ants (family Mutillidae), and cuckoo wasps (Chrysididae) have been recorded as parasites for different species of Bembecinus around the globe. Surprisingly, no fly parasites have been documented. Satellite flies in the family Sarcophagidae are insidious pests of most all other burrowing wasps, so this is a mystery.
There are currently ten species of Bembecinus listed for North America north of Mexico, collectively found across most of the continent. Approximately 190 species are known worldwide, on all continents but Antarctica. The specimens I was watching measured an estimated 8-10 millimeters in size, but some species are larger and others smaller.
I may have to go back and see if I can find more of these interesting sand wasps. I hear they are even active on extremely hot days, and we have had a lot of those this year in Colorado Springs.
Sources: Evans, Howard E. 1966. The Comparative Ethology and Evolution of the Sand Wasps. Cambridge, Massachusetts: Harvard University Press. 526 pp.
O’Neill, Kevin M. 2001. Solitary Wasps: Behavior and Natural History. Ithaca: Comstock Publishing Associates (Cornell University Press). 406 pp.