I have searched the internet and found many similar beetles (rose beetle, june beetle etc) but none so small and eating ONLY the polygonum which is generally supposed to be pest and disease free. What are they? They are no more than 5mm long and bright metallic emerald with black underparts. Antennae are fairly long, flexible and made up of a string of tiny blobs when you look with a magnifier. They do not fly away but drop to the ground and play dead till you've gone. Is it a foreign import that likes our hot summer - is it potentially part of the answer to Japanese knotweed - or is it just something common and nasty I just haven't noticed before?