The BugLady was enjoying the prairie flowers at Riveredge one day when she saw this odd little wasp that was also enjoying them (the flower is Rattlesnake master).  Turns out it was a Black Thorn-footed Mason Wasp (Zethus spinipes).  Mason wasps have appeared in these pages before (https://uwm.edu/field-station/bug-of-the-week/bramble-mason-wasp/) – let’s review who they are.

They’re in the family Vespidae, a family of almost 5,000 species (325-ish in North America) that includes social wasps and solitary wasps and lots of different lifestyles.  Paper wasps, yellowjackets, and hornets are its better-known members, with the potter and mason wasps (subfamily Eumeninae, which used to be its own family, Eumenidae) sneaking in under the radar.  The BugLady was surprised to read that although they are relatively inconspicuous, the Eumeninae make up the vast majority of the Vespids, with 260 species here and 4,500 species worldwide.  Ants, bees, and wasps are under a big umbrella, the Order Hymenoptera. 

The Eumeninae are solitary wasps that feed on nectar but serve up the bodies of other invertebrates – mainly moth caterpillars – to their larvae.  They have a variety of nest styles – some are opportunists that use pre-made holes or cavities mostly above ground, like tunnels of other insects, nail-holes, etc. and others build egg chambers from natural materials like mud, sand, or vegetation.  The name Potter wasp comes from the (very cool) urn-shaped mud nests constructed by some species https://www.bugguide.net/node/view/439141/bgimage.  When they’re finished, the egg cells are stocked with paralyzed bodies, an egg is laid (some species suspend the egg from the ceiling with a strand of dried fluid), the chamber is closed, and a new one is started beside it.  When the eggs hatch, dinner is served. 

Eumeninae is the biggest subfamily in the Vespidae, and Zethus is the largest genus in the Eumeninae, with about 250 species worldwide (and, of course, its taxonomy is under review).  They are mostly neotropical (Brazil is a hotbed of Zethus species), with only six species in North America north of the Rio Grande.  Of those six, five are restricted to the far southern portion of this country, and one is widespread in the eastern US (in short, the BugLady is sure of her ID on this one!). 

Zethus, weighing in at ½” to 5/8”, may use bits of vegetation or wood to make nests in twigs and branches, in used insect tunnels, or underground, but they don’t make mud nests. 

We think of wasps as being, well, “wasp-waisted,” and some take it to extremes, especially in a different wasp family, Sphecidae, the Thread-waisted wasps https://www.bugguide.net/node/view/2489169/bgimage.  The constricted bit – the stalk that connects the thorax with the rear of the abdomen – consists of the narrowed first (and sometimes second) abdominal segment.  This stalk is called the “petiole” (the abdomen is “petiolate,” the opposite of which is “sessile”).  Meanwhile, back in the Eumeninae, Zethus and another genus, Eumenes, have conspicuous petioles https://www.bugguide.net/node/view/2556597/bgpage, but at the back of the petiole is swollen area that ends in a second, wider petiole.

At last – the BLACK THORN-FOOTED MASON WASP!  There are two subspecies, one southern (Zethus spinipes variegatus https://www.bugguide.net/node/view/2397609/bgimage) and one northern (Zethus spinipes spinipes).  The northern subspecies is found in the northeastern two-thirds of the continent from the Atlantic to the Great Plains.  The wonderful, Minnesota Seasons website says that they are uncommon in that state, at the western edge of their range. 

The BugLady couldn’t find a specific explanation for the “thorn-footed” part of its name, though it does have a spine on each back leg where the tibia meets the tarsus.  Look closely https://www.bugguide.net/node/view/1873689/bgimage 

Not a lot is known about the reproductive (or other) biology of the BTFMW.  Females secure caterpillars with their mandibles and then sting and immobilize them.  They use or enlarge existing cavities in rotting wood or under bark, and they partition the egg chambers with wood fibers.  She may cap the tunnel with a cut piece of leaf.  Here’s a nice series of a female caught in the act of working on an egg tunnel https://www.bugguide.net/node/view/579657/bgimage. 

It is common in the Eumeninae for males to emerge from the egg tunnel before females, sometimes by a week or more, which means that when she oviposits the female must deposit eggs that will develop into females at the far end of her tunnel, and eggs that will be males closer to the front.  Magic?  You bet!!  A fertilized egg will develop into a female, and an unfertilized egg into a male, and she can decide, as she’s about to lay an egg, whether to release sperm that will fertilize it.

Because some of the moth larvae they cache in their egg cells are agricultural pests (like the Nantucket pine tip moth), BTFMWs are listed as a beneficial species.

FUN FACT: Like those of many other Hymenopterans, Eumenid larvae can’t poop – their digestive tracts aren’t “hooked up” until they metamorphose into adults.  This is probably an adaptation to their spending their larval stage in a “locked room.”  If they could defecate, it would be an unhealthy locked room.

Kate Redmond, The BugLady

Bug of the Week archives:
http://uwm.edu/field-station/category/bug-of-the-week/