Sand Wasps (Family Crabronidae)

Greetings, BugFans,

The BugLady has been hanging out on a sandy path at Riveredge, watching this pretty aqua-eyed Bembix sand wasp (possibly Bembix americana) go about its business. It’s sharing the path with myriad ants, some bee flies, and a few Punctured tiger beetles, and various wasps are working the lemon horsemint around the edges, and they’re fun to watch, too. These smallish (maybe ¾”) wasps make the Energizer Bunny look like a slug.

Sand Wasps

The name sand wasp is applied to some of the thread-waisted wasps in the family Sphecidae, and also to these digger wasps, which were formerly placed in the Sphecidae but are now in the Square-headed wasp family Crabronidae. They are (as are most wasps and bees) solitary wasps, found in habitats with loose or sandy soil. While they are not social insects like honeybees, ants, and some hornets are, they will tolerate other wasps nesting nearby.

Sand wasps—because they make tunnels in the sand to lay their eggs in. Excavation is accomplished using both their mandibles and their front legs—they chip away at larger chunks of soil with their mouthparts and then use spines on their front legs (tarsal rakes) to whisk small dirt particles off to their stern, and they dig fast enough, as one source pointed out, that they disappear from sight pretty fast.

There have been a number of BOTW episodes devoted to solitary bees and wasps, and their biographies are similar. The female excavates a tunnel and carves out one or more egg chambers, then she frequents the flower tops so she can supply each chamber with a heap of paralyzed invertebrates. She lays an egg, closes the chamber and leaves her offspring to consume the food cache and pupate within the chamber (alternatively, she places her egg in a chamber already provisioned by another insect, so that her offspring can eat both).

Bembix has a more “hands-on” parenting style. Like other Hymenopterans, the female’s digging is fueled by nectar that she sips from flowers, using a probe formed by a pair of fused mouthparts. Sand wasps practice progressive provisioning—the larder is stocked with one fly at a time, and when the larva has finished its first fly, the female delivers another and then another with increasing frequency until the larva is ready to pupate. Mother Bembix starts her offspring out with small, tender flies; the larva softens them even more with its saliva and then it eats the abdomen. In behalf of her young, she hunts for flies, eschewing the fragile, leggy mosquitoes, crane flies, gnats, and midges in favor of the beefier tachinid, horse, deer, and house flies, often catching them on the wing (a kissing cousin of Bembix carries prey back to her young impaled on her stinger). Unlike other wasps and bees that cache in their egg chambers paralyzed prey that stays fresh while it is eaten gradually, Bembix may kill the prey it delivers because it’s used right away.

It’s a lot of work! A tunnel may be ten or more inches long, and in addition to the tunnel that she’s actually supplying, she digs a few decoy tunnels to stymie predators/parasites. There’s only a single larva per tunnel, and one source said that she may have two active tunnels going at once. She recognizes her own tunnel because of landmarks like particular rocks, and she makes orientation flights daily to refresh her search image. Several hard rains that occurred during the BugLady’s observation period changed the landscape minutely, and the sand wasps were busy locating their digs.

A larva may require as many as 20 flies during its one-to-two week larval period. After each food delivery, the female camouflages the tunnel by filling it in. She emerges head first and walks away from the hole, scratching the loose dirt as she goes and sending it into the mouth of the hole to disguise it. Then she backs in, scratching and kicking the dirt deeper. Let Rau and Rau tell it, inWasp Studies Afield (1918):

Thus she continues until the hole is filled so near the top that she can’t back down into it anymore; then she assumes the same position backwards on the hole and crams and packs the dirt down with her hind pairs of legs and pounds, rubs, and punches it down with her abdomen; then brushes more dirt back upon the depression and repeats the packing process two or three more times, until the fill is brought up exactly level with the surface of the ground.

The Raus observed that females spend the night in tunnels, and they continue: “We have also found her here safely hidden away on dark or rainy days, for Bembix loves only warmth and sunlight. The tunnel is always temporarily closed under these conditions. This is accomplished by the Bembixgoing down into the hole and pushing up loose dirt from below until the mouth of the burrow is completely closed.”

Mature larvae eventually get closed in for good, forming a cocoon of silk and sand grains, and spending the winter as a pre-pupa. They finally pupate in spring and emerge in summer, and then the wild rumpus begins—Crabronid sand wasps stage mating dances called “sun dances”. In his blog, Bug Eric, Eric Eaton writes,

Males emerge before females, and fly erratically at a 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.

About the only way to get stung by a sand wasp is to grab a female (males have no stingers) or to step on one barefoot. They are pretty single-minded about their digging and will fly around an observer’s ankles harmlessly. According to the Missouri Department of Conservation website, they may be attracted to the flies that are attracted to people, and it’s possible to put a fly in your hand and have the sand wasp take it.

The BugLady