A difficult trio and some individuals will only be identifiable by comparison with a series of specimens.
B. bualei and B. tetracolum are the most difficult pair, so first identify B. femoratum as follows:
1a. Antennae more extensively dark: 3rd antennal segment clearly darkened for most of its length, 2nd antennal segment often also darkened. Femora normally extensively blackened. Smaller species: 4.2 – 5.2 mm.
Flat and pale like bualei but averaging smaller still and with darker legs and antennae. Elytral striae and wings like bualei.
Beware bruxellense in which the pronotal microsculpture can be hard to see, and which shares the dark legs and antennae of femoratum. The pronotum should still appear dull and the base is punctured like tetracolum.
1b. Antennae with more extensive pale bases: 3rd antennal segment at most with a slight darkening towards the apex, 2nd antennal segment entirely pale. Femora uniformly pale or slightly darkened towards base, sometimes overlapping with pale-legged femoratum. Two larger species: 4.5 – 6.1 mm.
2a. Pronotal base with weak punctures merging into longitudinal wrinkles, best viewed on the area between the basal foveae. Elytra flatter, the striae shallower and more weakly punctured. The striae are deepest at the middle of the elytra, fading towards the shoulders. Basal half of 7th elytral stria (view beetle tilted partly onto its side) represented only by a row of very small punctures, these not connected by a furrow. Size smaller. Elytral spots paler yellow-brown, the wings more easily seen through the elytra. Legs also paler, the femora sometimes with darker clouding towards the base, recalling femoratum.
bualei (formerly known as andreae)
2b. Pronotal base distinctly punctured, the puncturation not masked by any longitudinal wrinkling. Elytra more convex, the striae deeper and more strongly punctured. The striae are equally strong from mid-elytra towards the base, or deepest near the base. Basal half of 7th elytral stria stronger, with at least a few deeper punctures basally, or even with some punctures connected by furrows. Size larger. Elytral spots darker orange-brown, more opaque. Legs also darker, femora always uniformly coloured.
Date: 22nd April 2010, updated 6th March 2013. http://markgtelfer.co.uk/