Mating pair on buttercup flower (Ranunculus californicus). There were a large number of these swarming over and landing on the buttercup flowers near a brook running through a grassy meadow surrounded by (mixed) redwood forest. It was a warm, sunny afternoon. Images from the same time/place/population appear in posts with thumbnail images below:



Initially I believed these were ichneumonids since there are many (>14) antennal segments; there appears to be no costal cell (or the bounding veins are fused/thickened...and yellow); there are two trochanter segments; and the dorsal tergites of the abdomen appear more sclerotized than the ventral sternites (see here). Those are a few items I noticed while trying to run these guys through the keys in Goulet & Huber(1) and Borror & DeLong(2). BUT...I made a major mistake at the very start in both keys...going with suborder Apocrita instead of Symphyta!

Thankfully, Ross Hill promptly corrected me...pointing me to stem sawflies (Cephidae). They happen to share the characters I noted above for Ichneumonidae (except perhaps for the "tergites more scleroterized than the sternites" part). Indeed, Goulet states (on pg 101) that in Cephidae there's a "slight dorsoventral constriction between abdominal segments 1 & 2", and that "They are unusually slender for sawflies" and "to the untrained eye adults are very much like those of Ichneumonidae". That gave me a bit of consolation for my error :-)

After Ross's nudge, I studied the 1969 CIS publication "The Cephid Stem Borers of California (Hymenoptera: Cephidae)(3)" by W. W. Middlekauff and it became clear this was Cephus (see here for some details). Natalie McNear also used Middlekauff to key her post here, and noted that he remarked on the apparent affinity of Cephus clavatus for the pollen of buttercups, which was certainly the case for both this population and the one Natalie observed in Marin County.