You may want to isolate that newly-bought Easter lily for a few weeks before allowing it to join your other houseplants. It just may be harboring a common fly pest called fungus gnats. Their larvae are also fond of poinsettias, cyclamens, African violets and carnations. But don't blame the plants for the cause. Reasons for the infestation are more likely rooted in poor soil conditions, health of the plant and the indoor environment.
Although certain plants are not the cause of their existence, fungus gnat larvae prefer to dine on the fine hair roots of showy indoor plants like African violets, which can lead to root rot. Plants like carnations, cyclamens, geraniums and other foliage droppers can also succumb to the pest. Infestation signs include faded plant color, droopy stems and general loss in growth vigor.
The fungus gnat resembles a tiny mosquito and is easily identifiable by its grayish-black slender body. It breeds its larvae in rich, organic soil (particularly the peat moss kind) commonly used for house plants. Although the adult flies are a nuisance, it is their larvae that can damage the plants. The whitish semi-transparent black-headed larvae leave their slimy trail on the leaves as they feed on the fungus of decaying debris.
Over watering, poor drainage and allowing decaying debris to accumulate in the pot create conditions for attracting fungal gnats and their larvae. They will also lay their eggs in unsealed bags of soil which is no longer sterile. Plants that are stressed, subject to leaf wilt resulting in foliage loss and stunted growth, may also be infested. The lack of predators indoors that control fungus gnat populations may contribute to their accumulation in otherwise healthy indoor plants.
Letting the soil dry thoroughly between watering, providing good drainage and removing leaf litter from the pot can help prevent infestations. Use sterile, properly sealed soil. To discourage excess moisture and fungal growth, sprinkle 1/4 inch of sand across the top of the soil or apply a thin layer of sphagnum moss. The moss increases acidity to deter fungus. Because fungus gnat larvae may already exist in the soil, Isolate new plants at least three weeks before grouping them with your other houseplants. Caulk and screen windows before the weather turns cool, to discourage fungus gnats from moving indoors.
Treat seriously infected plants by applying pyrethrin plant derivative sprays on the soil surface. Aerosols and sprays easily kill the adult gnats. Luring female gnats with decoy pots of sprouting grain may discourage them from laying more eggs in or on your houseplants and is an alternative to applying pesticides. Before the larvae hatch, discard the grain pots and set out new ones.