Tiny black things

Hard to see in this picture but out of the blue I noticed that most of the underside of the leaves on my snow peas were covered in little black spots. At first I thought it might have been some kind of disease but given that they 'squish' a deep rusty colour I guessed they were some kind of bug. They were not on any other plant.....

In any event a quick squirt with the Pyrethrum and Garlic spray fixed them up.