Aphid Wasps – Subfamily Pemphredoninae

Subfamily Pemphredoninae consists of about 1050 species in 40 genera worldwide [1]. The pemphredonines are small to tiny sphecids, many of which display one or more of the following distinctive characteristics: A stemlike sternal petiole (the proverbial "wasp-waist"), a cuboidal head (many wasps in the parent family Crabronidae are commonly called "square-headed" wasps), and a disproportionate enlargement of the forewing stigma [2].

Other identifying characteristics:

• Antennae below or at midline of face, sockets contiguous with frontoclypeal suture.
• Compound eye inner orbits essentially parallel, eyes usually widely separated; ocelli normal
• Clypeus transverse, often very short.
• Pronotal collar short, usually high, often transversely carinate
• Mandible without notch or step on margin, inner margin simple or with teeth; mouthparts short.
• Midtibia with one apical spur; midcoxae nearly contiguous; claw simple; hindfemoral apex simple [2].

Pemphredonines are often abundant, but being rather small and adept at hiding (the pair pictured here were hidden underneath a leaflet when I spied and uncovered them), I don't see many of these, and I consider myself fortunate to have encountered a cooperative couple; it's usually futile to chase one of these wasps while they are nectaring or hunting, they just won't hang around long.

Female pemphredonines provision their nests with Homoptera (principally aphids, hence the common name), or in a few cases, Thysanoptera (thrips) or Collembola (springtails) [2]. 

Wasps in the tribe Psenini (psenins),  like these, are between 4 and 15mm long. All have a distinct wasp-waist, the aforementioned petiole made up entirely of the first sternum. Female psenins are more liberal in their choice of prey, using mostly cicadellids, fulgoroid, membracid, cercopid, or psyllid planthoppers. 

Pemphredonines are represented in the early fossil record in a find at Cedar Lake, Manitoba, Canada, of a specimen in the genus Lisponema in Upper Cretaceous (965-100 mya) amber [2].