Ignore the ants. They do not exist for the sole reason of milking and protecting aphids. When your biological balance is successful, you will see some ants, some aphids, and a wide variety of biological control agents, all somehow making a living but without damage to your ornamentals.

ignore the insectisidal soap. Take a good strong spray hose. And blast the aphids and under the leaves. Do it every day for a week. Apparently if you do it every three days your giving time for the aphids to breed.

i tried that on a rose that had aphids. For three days i sprayed it and under the leaves too. No more aphids after that. And i used a strong water blast hose. if you don't have a pressure thingy to attach to the hose, put your thumb in front of the hose to get water pressure and blast away.

I'm following up to my own message. Perhaps someone else will have the same issue. I tried spraying with water every day, but these are young plum trees with flimsy branches, and they simply swayed away from the stream. Didn't knock off many aphids. I resorted to holding each branch in my hand, while teetering off a ladder, carefully washing off the aphids. I soaked myself, and risked breaking my neck. I decided that this approach was folly, particularly since it had no effect on the aphid population.

I ended up controlling the ants with Tanglefoot, and that seems to have been the key. After controlling the ants, reducing the aphid population with insecticidal soap (about three applications over the course of a couple weeks) got things to the point that the beneficials can handle the load. Problem solved. Ants pointed elsewhere, few beneficials killed, and aphids under control.

If this were ornamentals, I wouldn't be that worried, but these are fruit trees, and the aphid infestation has been heavy enough that there were no blossoms this year.

What kind of insecticidal soap did you use? And do you have to use a special sprayer? I've got 4 citrus trees totally covered with aphids and ants. I don't want to use systemic control (obviously) but baits don't work, and neither does spraying the ant nests. Plus, the ants have invaded my house (they keep going for the kitchen/bathrooms). I'll see if HD carries tanglefoot. I'm glad it seems to have worked for you.

I used a couple different insecticidal soaps. Brand didn't seem to make any difference. I think it was Safers. The aphid population has gone through a couple surges since I posted this. I've squirted the leaves a couple times since, and things have remained manageable.

I do have a sprayer (10 bucks at Home Depot) which I used initially when the trees were completely infested, but I have since been using a little squirt bottle to stay on top of things. Easy to grab on the way out the door if I'm just trying to get a few when the beneficials don't seem to be keeping up.