The fungus gnat is a small, slender, fragile black or grey fly measuring
only about 2 1/2 mm in length, with rather long legs for its size. It is a
close relative of the sewer fly, which is slightly larger than a gnat, dark
brown to black in colour, and is commonly found in the bathroom and around
sources of water.
The adult fungus gnat lays tiny eggs, singly or in clusters, on the
soil in houseplants. In about 4 days, the eggs hatch into tiny, silvery
white larvae (resembling tiny earthworms), with shiny black heads. When
mature the very slim, and transparent larvae can measure up to 6 mm in
length. Within a 2-week period, the larvae begin to spin tiny silken cocoons
in the soil. The pupal stage lasts only 1 week. Adult fungus gnats emerge
and mate. The adult stage lasts only 1 week, but the female will lay around
100-150 eggs during that time. The entire life cycle lasts only 4 weeks --
from egg to larvae to pupae and back to egg.
Feeding - Fungus gnats seldom cause much damage to plants. The larva
(worm) is the only form to feed, and it feed mainly on fungus and organic
matter in the soil. When their numbers become high, the larvae will
sometimes feed on the very tender root hairs on the plant. Plants damaged by
gnats will lose vigor and may show more than normal yellowing of leaves, and
Control - One of the easiest ways to control gnats is to allow the soil
in houseplants plants to dry before re-watering. Larvae cannot survive in
dry soil. Gnats are most commonly found in soil that has a high percentage
of organic matter (especially peat moss). Instead of using peat moss in, try
perlite or vermiculite.
Insecticides have proven effective against fungus gnats. Diazinon (2%
dust) applied to the soil surface and worked in slightly will come in
contact with the larvae and kill them. Malathion (50% EC), at the rate of 2
ml per litre of water and applied to the soil surface is also effective.
Note that some plants may be damaged by these insecticide. Read and follow
Aerosol insecticides containing any of a number of chemicals (pyrethrins,
resmethrin, tetramethrin, and d-trans allethrin) can be sprayed into the
air. These insecticides must come in contact with the insect to kill it.
Spray the soil, not the foliage, which may be damaged by the insecticide.
Since these insects do not feed on the foliage, it is not necessary to spray
the leaves the leaves.
Since the life cycle of the fungus gnat is short, it is necessary to
apply an insecticide every seventh day, for 4 or 5 weeks. If an application
is missed, the insect may be able to emerge, breed and lay eggs.
Organic insecticides such as insecticidal soap and diatomaceous earth
have proven effective in controlling fungus gnats, although further testing
is required. Some plants may be damaged by these insecticides, therefore
read and follow labels carefully.
Brightly coloured traps have proven effective in some instances. One
suggestion is to paint a milk carton bright yellow, then coat it with a
sticky oil. Place or hang the carton near windows or where fungus gnats
congregate. When the gnats are attracted to the bright yellow colour, they
stick to the oil and die.