Orchids are rarely attractants to pests but ants can become attracted to the sticky residue on some orchid stems. Ants can also be attracted to the moisture in the orchid's potting medium particularly in dry climates or during times of drought when water is hard to come by. Orchids are very sensitive to chemical insecticides so cultural measures and physical traps and barriers are preferred to chemical intervention.
Flush your orchid plant and its planting medium with running tepid water at a medium to strong rate of flow for several minutes to wash out the ants. Run water down the bottom third of the stem first to prevent the ants from running up the stem. Squash any ants that climb up onto the stem or your hands during the flushing process. Repeating this process every two to three days for a week should put an end to a mild infestation.
Change out the orchid bark around the plant roots when flushing the ants with water is insufficient to displace them or they have laid eggs in the medium. Pull the plant from its pot and brush away all of the potting medium from the roots and discard it in the trash or compost pile outdoors.
Rinse the lower orchid stem and roots well in tepid water. Set the plant aside and wash the plant pot with hot water and soap and rise well to ensure there are no ants or eggs remaining. Replace the plant in the pot and fill in with fresh orchid medium snugly to stabilize the plant. Water in well to wet the medium and allow excess water to drain away.
Set commercial ant traps in the location where your orchids are kept. These will attract the orchids without causing any harm to the orchids themselves or the ambient air. Leave the traps in place for the recommended period or until the any problem dissipates. If you have animals in your home be sure to use animal safe ant traps.
Create a moisture tray for your orchid with a shallow tray filled with pebbles and water. Set the orchid atop the pebbles to serve as a moat to keep the ants at bay and also provide ambient humidity that the orchid will thrive in. Adding a few drops of liquid dish soap or mineral oil to the water can be an extra line of defense against the ants. Refill the water as it evaporates to keep the tray full at all times.