hmm ants do eat the larval stage of many insects, these ants i would welcome, if the ants are aphid milkers id worry about aphids. If they are a bother rather than using poision, try Tangle Foot, this product goes at the base of the stem and they cant climb over it. if your plants touch the fence however, this wouldnt work. if the ants are eating the plant, there are products like antex, or products that they take to the queen and wipe out the whole colony. there is a good chance they are a benefit to your girls though and poisions are poision
Ant go onto plants for a reason. Sometimes you might see one or 2 "on patrol" so to speak, looking for food. This is no problem
Othertimes, ESPECIALLY if you've got a "trail" of ants, they may be tending aphids, which they use like cows (they milk them for their sweet secretions). This is bad because aphids suck juices from the plants.
Insecticides work. Homemade pepper/garlic spray works great for this problem. A biological way to get rid of aphids is ladybugs.