The young are spiny and generally feed on the bottom of the leaves. The adults will be on the top and bottoms of leaves. The leaves become speckled white and blooms may be be reduced. You can spray them with insecticidal soap, Orthene or liquid Sevin. On the other hand, lantana will die back with the frost, so you may consider ignoring the problem.
Fire ants are a constant problem.
I heard another bit of interesting and information lately. If the fire ants are weakened by pesticides or other means, they may even take over. This sounds great. These smaller, non-stinging pests are bad to get in the house. At least fire ants stay outside.