For those of us who grow outdoors part of the year or in a greenhouse, ants are a nuisance in our orchid pots. In addition, they farm aphids, mealy bugs and caterpillars which can seriously damage tender new growing parts of the plant, especially buds and inflorescences. The honeydew which is left from these insects allows mildews to grow and provides another place for attacks on our plants.
The ants themselves do relatively little damage to the plants, but if you've ever picked up a pot with ants in it, especially fire ants in the southern United States, you know how the ant bites will feel to the plant grower! Spring and summer tend to be the times that ants are attracted to orchid pots. It is a great home for them -- they can move right in without digging through soil and removing it for their tunnels.
If the pot is seriously infected and the ants come swarming out when you try to pick it up, mix up a gallon of water with a cup of insecticidal soap or dishwashing liquid and pour about half of it slowly through the pot. It will not kill all the ants, but will kill any that it comes directly in contact with. If you have a spray bottle, spray the plant with the solution to kill any that escape the pot and swarm on the foliage. It is best to do this in a shady location or when the plant is in the shade since you don't want to allow sun to burn the leaves through the water. Allow the pot to stay in the same location for about an hour without disturbing it.
Put the rest of the solution in a bucket and fill with water. If it's large, then add more detergent. Pick up the pot carefully and submerge in the water. This will kill the rest of the ants as they come in contact with the soap. Leave in the water for about 15 minutes and watch out for any pests that come out of the media and climb up the foliage. Remove from the water and then drench thoroughly with water from a hose to remove all residue of the soap and to eliminate the chemicals which ants secrete when they make their nests. Be sure to allow the plant foliate to dry completely before returning to a sunnier location and the media to dry before watering the plant again.
Fortunately ants are more a nuisance to us than a danger to our plants. Removing them is relatively simple once you know what to do!