March 2016 update: Since writing this post in December 2011, it has become clear that “Trachys troglodytes” actually consisted of two species: Trachys subglaber and the true Trachys troglodytes. This post and the photo above refer to Trachys subglaber. For more info on Trachys, see the jewel beetle page.
While Trachys subglaber may not be quite as jewel-like as some of its larger relatives in family Buprestidae, it is still a little gem. I’ve only started finding it in the last couple of years while surveying calcareous grasslands with a suction-sampler.
The best way to record this species, as Keith Alexander has described (Alexander, 1989), is to look for the larval leaf-mines in the host plant: Devil’s-bit Scabious Succisa pratensis. Late summer into early autumn is a good season to be looking, when the Devil’s-bit is in flower. But if you are confident at recognising Devil’s-bit from just the leaves, the leaf-mines can be found from at least 7th June (my earliest record).
Trachys subglaber is not the only species to mine the leaves of Devil’s-bit but the shiny black spot is diagnostic; it marks the spot at which the egg was laid and thus marks the point from which the larva starts feeding to produce its full-depth blotch mine.
Until Keith sussed out the leaf-mines and published his note, Trachys subglaber was regarded as quite a rarity. The current map shows it is widespread in southern Britain, though not nearly as widespread as the distribution of its host-plant! [16.iii.2016: note that I have linked to the map of Trachys troglodytes on the NBN, but this is an error on the NBN – most of these are records or either troglodytes or subglaber and most should be reassigned to T. subglaber]
Alexander, K.N.A. (1989). Trachys troglodytes Gyllenhal (Coleoptera: Buprestidae) widespread in the Cotswold limestone grasslands of Gloucestershire. British journal of entomology and natural history, 2: 91 – 92. [Browse this article online here].