GNATS - are one of those pests where the life
stage that you see is not the one doing the damage.
They are mostly noticed on indoor plants and seedlings,
although they do also occur outdoors in the garden.
Fungus gnats belong to the family Sciaridae and
the adults resemble small mosquitoes about one eighth
of an inch in length. They are not strong fliers
and can sometimes be seen taking a break on the soil
surface. They are attracted to moist growing media
with high organic matter content, where they lay
their eggs. The eggs hatch into worm-like whitish
larvae up to a quarter inch in length with distinctive
shiny black heads. The larvae feed on the decaying
organic matter in the growing medium, but sometimes
attack the plant roots. A large scale attack can
result in stunting of the plant, and it is also thought
that these insects may play a part in encouraging
rot diseases. Avoid overwatering, and allow the soil
to dry out, especially on the surface, between waterings.
Sticky yellow cards are useful for monitoring their
presence and may effect some control. Biological
controls are often used, such as Bacillus thuringiensis
(BT), predatory mite (Hypoaspis) and beneficial
nematodes Steinernema or Heterorhabditis.
To monitor for the presence of the larvae place a
few peeled raw potato slices about a quarter to half
inch thick on the surface of the growing medium for
several hours. This will attract the larvae and give
you an indication of how extensive the infestation
is and whether or not your control measures are working.