Looks like you have a nice little aphid colony going there- the waxy aphids look like that white powder you described.  The ants are attracted to the sweet honeydew that ants exude when feeding.  Often times, you will see a black sooty mold develop later on that is an indicator of aphids.  Unless more than 1/3 of the tree is affected (leaf loss) there is no harm to the plant.  More cosmetic than anything.  And if you leave it alone, it gives natural predators a chance to move in and have a nice lunch.