Fruitfly,

Clearly what you have is a bad infestation of some kind of aphid.  From the photo, I can't tell what type of aphid it is but -- like many other aphids -- they are often literally "farmed" by ants who eat the "honeydew" (--i.e., "sugary poo") that the aphids produce while feeding on your tree.  

There are various things that you can spray directly on the aphids to kill them, such as insecticidal soap [like "Safer's i.s." brand], but you'll also have to dissuade the shepherding ants by something like putting a band of "tangle-foot" around the base of the trunk.  "Tanglefoot" is kind of a sticky, oily or greasy, substance that you put in a band on the trunk around the base of the affected plant.  Any shepherding ants trying to cross that line will get stuck in it and die, thus eliminating the ants who "tend" the aphids and move them about.  

My suggestion is that you go with the least toxic response that is still a functional response to the problem.  One doesn't need to "bring out the big guns" for the situation.  Once you put the tanglefoot around the base of the infected tree, be willing to gooey-up your fingers by squishing the aphids with your fingers.  

Reinettes.