Sand Wasps, Genus Bembix

Solitary wasps are among the most easily observed of insects, and their behaviors and life histories are intensely captivating. Sand wasps in the genus Bembix are familiar and common throughout North America, digging their burrows in dunes, on beaches, and other habitats with loose, deep sand.

Bembix female, New Jersey

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.

Bembix female, Lathrop State Park, Colorado

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.

Bembix male, southern California

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.

Bembix female on saltcedar flowers, Colorado

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.

Bembix in Colorado

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).

Wasp Wednesday: The "Horse Guard"

Heidi and I were away last week in Cape May, New Jersey for a belated honeymoon, celebrating in the same place we had our first “date” back in October, 2010. We went a week earlier this time, and visited some locations we had missed on our last trip. Consequently, we saw some different birds and insects, including a wasp that was on my “bucket list.”

The Triangle Point Butterfly Garden, a tiny park near Cape May Point State Park, is planted with a variety of native and exotic flowers that attract many kinds of insects. We visited in the early afternoon of October 5, and found another party looking at the butterflies, but wondering what the big black and white wasps were.

”Oh, my God, that’s a Horse Guard!” I couldn’t contain my enthusiasm as I frantically focused my zoom lens on the wasp. I had forgotten that Stictia carolina, the largest of our “sand wasps” aside from cicada killers, ranged this far north, and persisted into the autumn months.

The Horse Guard is found from New Jersey and Pennsylvania south to Florida and west to Illinois, Kansas, and New Mexico (barely). It is most abundant in the southern Great Plains. Though solitary, each female excavating her own nest, many wasps may nest in a small area of sandy soil.

How did the Horse Guard get its name, you might ask? Also known as “cowfly tigers” and “insecto policia,” the wasps are specialist predators that chiefly attack horse flies. Naturally, the best place to hunt horse flies is around equines. The female wasps will fly around a horse, up and down each leg, searching for their prey. They can even fly backwards in front of a moving horse. This hunting behavior is frequently interpreted as aggression by “hornets” by the average, intimidated horseback rider or ranch hand. Once people understand the intent behind the wasps’ persistent hovering, the usual reaction is one of relief and elation.

”That’s fantastic! How can I encourage more of these wasps for horse fly control?”

Short of importing a sand dune, as the late Howard E. Evans writes in his book Wasp Farm, there is very little one can do to ensure the presence of horse guards at any given farm or ranch. The best thing to do is simply not kill them or accidentally obliterate their nesting areas.

Males search for females by engaging in “sun dances,” which are level flights in circles, figure eights, or sinuous patterns, occasionally perching on low herbs, dung, stones, or on the ground. These patrol beats are most often in the vicinity of female nesting areas, and usually in the morning hours. They will actively chase each other, or molest other large insects that pass through their individual territories.

Females take a surprisingly long time to complete a burrow, taking an average of 22-30 hours. Burrows are usually initiated in the morning, with frequent breaks as the sun becomes more intense, with digging activity increasing again in the late afternoon. The wasp closes the tunnel each time she leaves the vicinity. The burrow is a diagonal excavation, averaging 35-51 centimeters in length, and to a depth of 18-24 centimeters. Soil properties influence these numbers. The tunnel terminates in a single cell, where the wasp deposits a single egg.

Once the nest is completed, the wasp begins hunting, feeding her larval offspring in the progressive fashion that birds do. Between visits, the wasp makes both an inner closure that seals off the terminal cell, and an outer closure that obscures the nest entrance at the surface. Fifteen to thirty-five flies may be fed to the average wasp larva during its lifetime. Larger flies (like horse flies), mean fewer flies are necessary to feed a growing larva. Still, the hunting tactics of the wasps can be very effective in protecting livestock from blood-sucking flies.

The double closure of the burrow between the frequent visits by the mother wasp helps eliminate many of the opportunities parasites usually use to gain entry into sand wasp nests. “Satellite flies” in the family Sarcophagidae (subfamily Miltogramminae) do deposit their own tiny larvae on the prey carried into the nest by the female Horse Guard, but those larvae are generally content consuming the leftovers of prey, rather than attacking the wasp larva itself.

Interestingly, while I was employed by the Cincinnati Zoo in 1988, we received pupae of horse guards from Mississippi that yielded both the expected wasps, but also “Cow Killer” velvet ants, Dasymutilla occidentalis. This would imply that occasionally the Horse Guard offspring fall prey to larvae of velvet ants.

There are two other species of Stictia in the southern U.S., but even more in the New World tropics (28 species total).

Look for the Horse Guard if it ranges in your part of the U.S. All the specimens imaged here are females, by the way. Males, active much earlier in the season, have the white markings confined to the front half of the abdomen only.

Sources: Bohart, R.M. and A.S. Menke. 1976. Sphecid Wasps of the World: A Generic Revision. Berkeley, California: University of California Press. 695 pp.
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, New York: Comstock Publishing Associates (Cornell University Press). 406 pp.
Pulawski, Wojciech J. 2011. “Catalog of Sphecidae sensu lato,” California Academy of Sciences

Wasp Wednesday: Bembecinus

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.

Wasp Wednesday: Bicyrtes

Among the most commonly encountered members of the “sand wasp” guild are the eight North American species in the genus Bicyrtes (By-SIR-teez). Two species in particular are widespread in the U.S. and adjacent southern Canada. They are much more sedate and easier to approach than most other members of the subfamily Bembecinae (family Crabronidae), and their predatory habits earn them some respect as well.

Female Bicyrtes actively hunt the nymphs of stink bugs in the family Pentatomidae as food for their larval offspring. The next time your child turns up their nose at Brussels sprouts, let them know it could be a lot worse. Actually, the accepted prey of these wasps includes equally odorous leaf-footed bugs (Coreidae), and shield bugs (Scutellaridae) and assassin bugs (Reduviidae) have also been recorded.

Wasps that burrow in the soil are termed “fossorial,” and a female Bicyrtes is an excellent digger. She can nest in soil of a more coarse texture than most other sand wasps. I imaged this Bicyrtes quadrifasciata excavating a nest in South Deerfield, Massachusetts in fairly rocky soil on the site of a former pickle factory, long since razed but with pavement fragments and pebbles in abundance.

While many sand wasps nest in dense aggregations of dozens (if not hundreds) of individual females, I find that Bicyrtes tends to be more of a loner. The wasp uses a “tarsal rake” of spines on the segments of her front tarsi (“feet”) to tunnel about 15-20 cm down into the soil at a shallow angle. Depending on the species the burrow terminates in a single cell, or several cells branching from the main tunnel. She closes the burrow while she goes off hunting prey.

When returning from a successful hunt, the wasp uses subtle landmarks to relocate her hidden nest. Meanwhile, we can’t remember where we parked the car.

Most sand wasps practice “progressive provisioning,” feeding their larval offspring as they grow, but Bicyrtes stockpiles prey, laying an egg on the first victim to go into a given underground cell. Once she completes a nest, she sets out to repeat the process, having cached enough food ahead of time that she needn’t hang around to spoon feed her children.

Both male and female Bicyrtes are very fond of nectar, and become distracted enough at flowers that they can be closely approached. Look for them on the blooms of Dogbane (and the related Indian Hemp) as well as milkweeds, white sweet clover, wild carrot (Queen Anne’s Lace), and a variety of other weeds.

Bicyrtes can easily be told from similar wasps by the very boxy appearance of the back of the thorax. The angular hind corners of the thorax are often white. Plus, if you are able to get close enough to see that character, you are looking at something other than sand wasps like Bembix and Steniolia that stick around for about a millisecond before zipping off to parts unknown.

I leave you with a parting shot of a female Bicyrtes from Bentsen Rio Grande Valley State Park in Mission, Texas that I have yet to identify to species. Maybe one of you can solve the mystery. In any event, keep an eye out for these wonderful wasps that stick it to our stink bug foes.

Sources: Evans, Howard E. 1966. The Comparative Ethology and Evolution of the Sand Wasps. Cambridge, Massachusetts: Harvard University Press. 526 pp.
Krombein, Karl V. et al. 1979. Catalog of Hymenoptera in America North of Mexico. Washington, DC: Smithsonian Institution Press. Vol. 2, pp 1199-2209.