Most probably you have got aphids. Flies that you see around are freshly born. They fly around and spread new eggs. Take a carefull look at leaves and you will see small animals, get closer with your finger, they will start to run. White stains are shit of these little creatures.
I use very effective spray, kills them instantly when it sits on plant, but I have been chasing them for some time and they are still present in smaller quantities. They do not propagate and move very fast, but are persistent.
When I was buying my spray, I was offered organic stuff too, but was warned that it works slowly - 14 days instead of 14 seconds.
If you do not get rid of them, you will see them on your developed buds later on and will not be pleasant at all.