The ants enjoy the honeydew the aphids secrete, and in exchange, protect the aphids from their natural enemies. First, you eliminate the ants so that they can't protect the aphids, by applying a sticky barrier around the base of your trees and shrubs. Then wash the ants AND the aphids off your plants with a stream of water from a garden hose. This simple treatment, repeated 2 or 3 times a week, can keep your plants nearly aphid-free. For a serious infestation, spray your plants with insecticidal soap, but be sure to read, and follow, label directions.
Spider mites are another garden scourge.
They seem to be particularly fond of junipers and arborvitae, although they can infest other plants, especially marigolds. These insects are about the size of a grain of salt, and their color varies depending upon the species and their diet. Since they are so tiny, they are hard to see with the naked eye. The best way to detect them is to hold a piece of paper under a branch you fear may be infested, and then to shake the branch. Spider mites feed on the underside of leaves, sucking out the plant juices, and causing the leaves to turn yellow and the plant to wilt.