Pirapion immune (Kirby, 1808) and Protopirapion atratulum (Germar, 1817)

Previously both were included in Pirapion, atratulum was separated into Protopirapion in 1990 by Alonso-Zarazaga. Gonget however dismisses this and believes both should be retained within Pirapion.
Both species have characteristic, pear shaped elytra, slightly narrower in P. immune especially so in the male. Both are black with sparse white setae throughout. Similarly small sized; P. atratulum at 2.2-3.1mm is slightly larger with P. immune at 2-2.8mm. As with many Apionidae the males in both species have a slightly shorter rostrum, the 1st tarsal segment in male P. immune has a distinct ventral tooth whereas the female and both sexes in P. atratulum are unarmed. Both species are normally found on Broom (Cytisus) but P. atratulum will occasionally feed on Gorse (Ulex).

Pirapion immune

Protopirapion atratulum

These two beetles can be separated reasonably easily using external characters although I'm not sure you could do it in the field , I can't but I'm an old git, you would need a good lens and an accommodating beetle.

Head sculpture
P. immune: Clearly striate between the eyes (frons). Coarsely punctured throughout including behind the eyes (vertex)
P. atratulum: Not striate between the eyes. Frons thinly punctured. Vertex unpunctured but strigate i.e. with faint but discernable transverse lines.

P. immune

P. atratulum

Pronotal Furrow
P. immune: Medial basal furrow very short, quite often absent entirely.
P. atratulum: Basal furrow much longer though usually narrower often extending as far as or beyound middle of pronotum.

P. immune

P. atratulum