Weird bug on a plant, never seen before, hundreds of them

I think they are Cantharidae - Soldier Beetles. 
I am guessing you are in the USA because I have seen very similar if not the same beetles.

Greetings,


I would agree with Paul K - most likely Chauliognathus pensylvanicus - assuming these images were taken in the eastern half of the U.S. There are over a dozen species of Chauliognathus in the U.S. with all but pensylvanicus and marginatus occurring in the western half of the country. In the East, marginatus and pensylvanicus can be separated easily by color pattern, especially in the form of the black marking on the pronotum. Additionally, C. marginatus is typically an early summer beetle, in flight through July, and pensylvanicus shows up later in August and flies well into the fall - being especially common on the flowers of goldenrod.

Interesting side note - the name 'pensylvanicus' is one of a number of cases where it is correctly spelled with only a single 'n'. I see these names often misspelled by folks with good intentions "correcting" them to read "pennsylvanicus", but a check of the original description will show that one 'n' is the correct spelling.

Cheers!
Bandrow