Answer: Ants do not usually cause a problem unless they are nesting in the roots. Usually, we see ants when there are aphids. Aphids do damage plants; they are small soft bodied sucking insects. The ants will be attracted to the sweet plant juices oozing from spots the aphids have damaged. A black mold can also grow on the juices and look like dots. If this is the case, your best bet is to control the aphids. They can be knocked down with a strong spray of water from the garden hose, or sprayed with insecticidal soap according to the label instructions. Aphids may be attracted to plants that have been oversupplied with nitorgen because this causes overly soft new growth, so make sure you are not overfertilizing. Also, aphids tend to be worst in the early part of the season and naturally slow as the summer progresses.
Another possibility, if there are overhanging trees, is that the trees are being attacked by an insect and dripping sap, again causing the mold on the plants below. You might look into that as well.
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