I was walking around Fitzwilliam Country Park this afternoon when I spotted lots of metallic black/blue beetles on the leaves of an alder. I think the alder is Italian alder (Alnus cordata).
I believe that the beetles are Alder Leaf Beetle. This species has been absent from the UK for about 60 years and it began to be recorded again in about 2004. It is likely that it was reintroduced to the British Isles by the plant trade.
There were many beetles on this and other trees, singles and mating pairs. You can see holes in the leaf in the picture above. This species causes significant damage to alder and can reduce leaves to skeletons. It was recorded at RSPB Old Moor in 2012 and it has been recorded at Wintersett since 2013.