Adult fungus
gnats are primarily a nuisance pest, although some damage can be done to plants
due to larval feeding on root hairs. This
type of damage can be serious in greenhouses and nurseries.
Fungus gnats
thrive under moist conditions, particularly where there is an abundance of
decaying vegetation, algae, and fungi. They infest the soil and container
media, specifically those rich in organic matter. Fungus
gnats do not bite people or animals and are not known to carry human pathogens.
They have a beneficial role as decomposers helping to convert dead
vegetation into nutrients for plant growth.
The adults are
dark in color and similar in appearance to mosquitoes. They are about 1/16 to
1/8-inch long, although they can be larger. The gnats are relatively weak
fliers and are usually not found flying around indoors.
Females lay
tiny eggs in moist organic debris or potting soil. The immature, legless larvae have a white to
clear body with a shiny-black head and when fully grown are about 1/4 inch long.
The larvae live in the soil and feed on organic mulch, leaf mold,
grass clippings, compost, root hairs, and fungi.
Control
Cultural and
physical control includes reducing moisture as much as possible, eliminating
any plumbing or irrigation system leaks and providing good drainage. Moist
grass clippings, other organic mulches and composts are favorite breeding
spots. The use of partially composted matter in potting mix will encourage the
infestation of gnats.
Insecticides
are used in commercial plant production but are seldom warranted to control fungus gnats around the home. Bacillus thuringiensis subspecies israelensis
(Bti) applied to the soil or potting soil is a safe product for the control of
fungus gnats. Repeated applications are needed for long-term control. Note: This
particular Bt product is different from the one used to control caterpillars.
Bt labeled for caterpillars is not effective against fly larvae. Insect growth regulators (kinoprene, cyromazine)
applied to the potting mix can also be effective.
Pyrethrins
or a pyrethroid (bifenthrin, permethrin) are an option to provide temporary,
fast-acting control. Pyrethrins have low toxicity to people and pets. When spraying
houseplants, it is best to move the plants outdoors for treatment and wait a
day or so before bringing them back inside.
Biological
Control - Predators,
such as some beetles, mites and nematodes will help to control fly larvae
outdoors.
