Fungus Gnat Damage
Fungus gnats are
most damaging to seedlings and freshly stuck cuttings; they are
unlikely to harm established houseplants unless the population
is extremely high. They are most problematic in our homes
through the winter and early spring. These native insects are
common outdoors, and we often bring them into our homes when we
take plants outside for the summer where they become infested,
and then bring them back indoors in fall.
One of the most important
factors in getting fungus gnats under control is managing soil
moisture. Fungus gnat larvae require a moist environment, and
may die if you can allow the soil to dry out thoroughly. Avoid
overwatering, and do not allow plants to sit in saucers filled
with runoff water. If possible, allow the soil to dry out pretty
thoroughly between waterings. Never take it to the point where
the plant wilts, though. You can also repot plants into fresh
potting soil, which will get rid of existing eggs and larvae.
Covering the soil surface with a layer of sand discourages
egg-laying by adults.
There is a Bacillus
thuringiensis var. israelensis (Bti) formulation sold under the
trade name Gnatrol that controls fungus gnat larvae. It is used
as a soil drench, meaning that you water the plants with it.
Ultra-fine horticultural oil, neem and insecticidal soap may be
sprayed on the foliage to control the adults.