Watch the bottoms of pots when you are shopping for plants. If you stand for a second you will see them in a nice little line coming and going from the pots they've set up homes in. One local big box store was so bad with them for awhile I quit shopping there. If an orchid has ants, usually you also have scale and a couple other things. Ants protect the other little buggies because they produce a sweet sticky goo that sticks to the leaves and ants love to eat it. Basically; avoid plants with pests....i've learned the really really hard way that it'll cost you your healthy plants.
Now that you have them...I hear ant traps sat around your plants helps keep them down. I'd never heard the raid thing...may have to try it on my shelves from time to time. On here you learn something new everyday!