This may be old news but I thought I'd share anyway.
http://www.ghorganics.com/page11.html
Ants themselves don't do much damage to your plants, however they protect and farm herds of aphids, cottony scales, mealybugs, soft-type scales, and whiteflies. Ants save the eggs of these insects in their nests over the winter and bring them out in the spring. These insects produce honeydew while feeding on your plants which the ants just love. If you closely observe ants streaming up a tree and tending their aphids, you will see them take their legs and rub the aphids which stimulates them to produce the honeydew. This activity is known as "milking" the aphids. The honeydew that is secreted by the insects favors the growth of sooty mold. This is a very destructive black fungus that spreads on plant leaves. The aphids are also responsible for spreading many different viral diseases.
http://www.ghorganics.com/page11.html