There is no reason to kill ants that are on your plants unless you see them doing some kind of damage. Plant damage from ants is not unheard of. Your best recourse, however, is not to apply pesticides to the plant, but to use a BAIT (like Amdro) to tempt the ants. They will take the baited product to the nest, feed it to the rest of the inhabitants and eventually the queen, and voila!