Fire ants protect pests like scales for the honeydew that they secrete they don't eat the scales. You will first need to elimate the ants in order to treat the scales. Ortho fire ant killer is a great "contact" killer that smells terrible but is very effective. The catch is after you sprinkle your gonna wanna irritate the ants to get them to swarm over the solution. IMO, I would not use anything that gets taken back to the nest since the nests appear to be in your plants and I would worry that those chemicals would mix with your watering cycle accidently poison your plants as well. But again, thats just my opinion. The ortho works very well and if you spot any mounds outside or inside sprinkle this on them as well. You may need to clip orchid roots hanging over the pot just a little so that they don't touch the table or the treatment. So long as the scales are protected by that many ants they will be almost impossible to eliminate completely.