Which beetle is this?

Answer

This is (as far as a photo can lead to a correct identification) the Poplar Rooster. As a scientific name for this species of beetle, you can currently find at least three different names in the literature: Melasoma populi, Melanosoma populi, Chrysomela populi. Unfortunately, it is not clear to me with certainty which name is currently accepted as the valid scientific name and which two are then synonyms. Given this ambiguity and given that Chrysomela populi Linnaeus, 1758, was described as a species by Linnaeus in 1758 (the ‘beginning of the era’ in terms of scientific naming), it is appropriate to keep this name at this time.