Many insect species have such subtle differences that it takes an expert with a microscope to tell them apart. The Western Ash Borer ( Neoclytus conjunctus) didn’t get that memo.
I found dozens of these colorful beetles with fleur-de-lis markings scuttling over a downed log just south of the White Oak Pavilion on one of the first 70-degree days of the year. Some had yellow patterns, some had white, and some individuals were only half as large as the majority.
Beetles aren’t generally social, so it’s not easy to compare specimens while they’re living, but I caught this group on a mass mating day. There were enough of them that they made quite a bit of noise. Their scrambling sounded like beads tumbling in a rain stick.
Because most of them were mating, I could tell the sexes apart. There were males and females with both yellow and white markings. I believe the smaller ones were only white, but I think those were also both male and female.
One explanation for the size difference is that smaller beetles didn’t get as much food when they were larvae, as Eric Eaton points out concerning another species of this Clytini tribe longhorn family beetle (see reference, below).
Female Neoclytus conjunctus lay eggs under the outer bark of dead and dying hardwoods, including oak and madrone. Their territory on the iNaturalist map seems to hug the I-5 corridor from B.C. to northern California, then fans out east and west of the Central Valley.
Sources say this beetle tribe mimics wasps. The side view above shows some convincing evidence—their striped bodies would be visible in flight. They also have back legs that resemble a grasshopper’s. Those impressive femurs, I discovered, are useful in battle. I took some video and caught a bout of leg-wrestling between two males, one trying to usurp the other’s female companion (see the screenshot photo above). The funny thing was, in round five of seven, the female turned sideways and left the ring. The two males kept fighting for a while, but neither one “won” the lady.
Eric Eaton on the related Banded Ash Borer: http://bugeric.blogspot.com/2012/02/. Accessed 4/5/21.
Scholarly article from the Journal of Economic Entomology on Neoclytus genus beetle pheromones. Includes some general information: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6281368/. Accessed 4/5/21.