There seem to be tiny little black bugs covering one of my cosmos, and it is slowly withering away. I have one plant in one trough and then another cosmos plant in a different trough which seems to be fine. I've noticed today that these little pests are starting to gather on some of the other plants near the affected cosmos.

Could anyone help with ideas on how to get rid of them/what they are? I do have a couple of photos, but didn't want to attach them here as I tried for a close up and they're fairly large, nearly 1MB.

Thanks in advance for any advice!