Fungus gnats are a common houseplant pest that seem to appear out of nowhere and can be hard to get rid of. During the winter when potting Amaryllids and bringing them out of dormancy they can be a perfect host for this annoying bug. The warm temperatures we keep our Amaryllis bulbs at and the waterings we provide to break their dormancy and grow, provide a great breading ground for fungus gnats.
Usually you won't know that you have fungus gnats indoors until one day when you see the little flies near a window or around your houseplant. These are the adults and while you may swat at them to kill them there may be plenty of larvae in the potting soil and they're continuously reproducing. Fungus gnats reproduce in moist and dark areas rich in organic matter where the larvae will feed on fungi and decaying organic matter. This normally isn't a problem but they will also feed on plant tissue, feeder roots and root hairs of potted Amaryllis bulbs weakening and potentially killing your bulb.
Once you've potted your Amaryllis bulb consider top dressing the soil. In the photo on the left I've top dressed the soil mix with inexpensive fish tank gravel. The benefits of top dressing your potted Amaryllis bulbs are more than just aesthetic. The gravel will help prevent fungus gnats from identifying your potting mix as an ideal area to lay their eggs. The layer should be at least an inch deep to diguise the moist potting soil below and be effective.
Other materials like sand would also be equally effective, as would a product called Turface that is used to amend lawns. Glass beads and large stones would probably not be as effective because they are larger and would give easier access to the moist potting soil below the surface.
An added benefit of top dressing your potted indoor bulbs will be visible when you begin normal watering routines. You'll see that less potting soil is accidentally washed out of the plant's pot and when watering from below the soil won't float to the surface of the pot and spill out. When and if you have any spill over it easier to clean up a few pieces of gravel than wet potting soil.