Aphids

Without a close-up of the insects, but based on the numbers, congregation, and color of them, these are Aphids. These small sap-sucking insects reproduce very rapidly. You will note larger and smaller ones. The larger ones are the females. They are born pregnant as are all their daughters. These insects are “farmed” by ants, so if you see ants on the plant around them, you’ll want to control the ants too. The ants protect the aphids from their natural enemies (like ladybeetles and other predatory insects) and eat their sticky honeydew secretions. Aphids may be controlled with hard water sprays to knock them off, and insecticidal soaps or horticultural oils. We’ve included a web link with more info to help you control the Aphids (and any ants). Use caution and always follow the label directions when using any kind of pesticidal product.
http://www.ipm.ucdavis.edu/QT/aphidscard.html
http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7411.html
(scroll down to the section on Trees and Shrubs for control of Ants on plants)