|There are four species of Phaedon listed in the British Isles.
P.concinnus Stephens is a coastal species limited by the occurence of its foodplants sea plantain (Plantago maritima)
and sea arrowgrass (Trigochin maritimum) and is unlikely to be found in the Watford area. The adult beetle
is bright metallic green but a rare metallic blue form could be confused with P.cochleariae (Cox)
in this case dissection may be neccesary, the aedeagus of both species are pictured in Joy.
The three remaining species are common and widespread. P.tumidulus Germar can be separated from P.armoraciae Linnaeus and P.cochleariae Fabricius by the disc of the pronotum which is only very finely punctured, appearing almost impunctate, whereas in P.armoraciae and P.cochleariae the disc is distinctly punctured.
P.armoraciae and P.cochleariae can be separated by the fifth elytral striae. In P.armoraciae it is deepened at the base outside of which is a distinctive humeral tuberosity. In P.cochleariae both of these characters are sometimes apparent but to a much lesser degree and normally absent.