Orchids are rarely afflicted by insect pests except in the case of fungus gnats. These gnats are symptoms of improper orchid care. The small flying insects and their larvae feed upon decaying organic material in the orchid plant's potting material, and also on the plant's roots. If they're not kept in check, the gnat population can explode and become both a nuisance and a health hazard to your plant.
Change your orchid plant's potting material. Only use material that decomposes very slowly, such as coconut fiber and charcoal, and change this regularly every 2 to 3 months. Alternatively, use a material that is inorganic, such as perlite. This removes the decaying material on which the gnats feed.
Reduce watering as much as possible, as the gnats and their larvae prefer a humid and moist environment. Always let the orchid plant's potting material and roots dry out between watering.
Avoid overfertilizing, as this further encourages gnats to congregate on your orchids. Nutrient needs vary widely by orchid species. Consult the nursery or orchid fancier from which you purchased the plant for species-specific fertilization requirements.
Trap and kill the gnats if the preventative strategies do not sufficiently reduce the gnat population on your plants. Use the yellow fly traps used to capture flies and aphids. Setup the traps adjacent to your orchid plant, or hang them from your orchid's leaves. The gnats are drawn to the yellow color where they become stuck and die.