Stress Encourages Pest Problems
Ants are usually a secondary problem on plants, more a nuisance than a serious threat. Typically, by the time ants infest a plant in sufficient numbers for you to begin to see them, the plant has experienced some degree of stress which made it vulnerable to the primary problem--invasion by serious pest insects.
The struggle of a plant against environmental problems such as drought, age, or excessive fertilizer or pesticide use makes it produce glutathione a chemical that seems to actually benefit certain harmful insects such as aphids, mealybugs and scale. It aids their reproduction, growth, and may even bolster their ability to resist pesticides. Pest insects prefer to attack these weakened plants that are struggling and lack vigor. Ants opportunistically rally to these infested plants because that is where the honeydew is.
NOTE: The presence of ants does not always indicate plant stress or pest infested plants. In the case of peonies, the presence of ants roaming the swelling, rounded buds signals all is well.
Some Causes of Plant Stress
After you deal with the ant problem, deal with the primary problem. Try to figure out what might be causing the affected plant to be stressed and attractive to pest insects in the first place:
Too much or too little sunlight or water.
Inappropriate climate or local conditions.
Excessive use of nitrogen-rich fertilizer.
Natural loss of vigor due to aging.
Temporary spring pest population explosion prior to emergence of beneficial predators.
Preventing Ants Next Year