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Pest Update – The Fungus Gnat

August 14, 2008  By Sean Valk



Sept 3, 2008  |  by Sean Valk

The Fungus Gnat
Growers often class fungus gnats as a nuisance rather than a pest, believing that fungus gnats will not damage their crop and that they only live for a short time. The problem is that ornamental crops can be impacted by both the adult and larval stage of the insect.

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Fungus gnat lifecycle. Fungus gnat adult.


 Fungus gnat larvae feeding on root.

While the adults only live for about 10 days, they can lay in excess of
200 eggs during that time. The life cycle begins with the eggs which
are laid  hatching into larvae, 5 to 6 days after being laid. For 12 to 14
the larvae feed and molt   The larvae are clear to creamy-white and can
grow to about 5 mm long with shiny black head capsules.  At the end of
12-14 days the larvae make their way to the soil surface to pupate.  The pupal stage may last 5-6 days.  The whole life cycle is completed within 4 to 5 weeks.

While the adults only live for about 10 days, they can lay in excess of 200 eggs during that time. The life cycle begins with the eggs which are laid  hatching into larvae, 5 to 6 days after being laid. For 12 to 14 the larvae feed and molt.

The larvae impact the crop by feeding on tasty root hairs and can even
work their way up into the plant stem.  The feeding also opens up
potential infection points for soil borne diseases.  Signs that a plant is
under attack from fungus gnat larvae may include a lack of vigor, wilting, yellowing leaves and leaf drop.  Root damage may appear as small brown scars on the surface.  Adult fungus gnats can also vector
disease by carrying spores of Pythium, Botrytis, Verticillium,
Fusarium and Thielaviopsis as they moved from plant to plant.
Spores have also been found in their droppings.  Growers should
protect their crops from fungus gnat damage in order to maintain high marketability.

Fungus Gnat Control – Clean, Dry and Larvicide

Cultural Control

Cultural control options include maintaining a dry, level, weed-free, well-drained area around your crop to help eliminate breeding areas.  Keeping compost piles away from the greenhouse, and cleaning up any spilled media on benches and the floor also helps eliminate breeding areas.   

Biological Control

Bacillus thuringiensis var. israelensis, trade name Vectobac®, is most effective against the young first instar larvae. The bacteria must be ingested by the larva, after which a toxic protein crystal is released into the insect's gut.   Repeat applications and variable rates may be needed to provide effective control.  Vectobac® is applied as a drench to the growing media.

Steinernema feltiae is a beneficial, insect killing nematode that are also applied as a drench treatment against fungus gnat larvae.  After entering the target insect through various openings, the nematodes multiply within the host and release a bacterium whose toxin kills the larvae. 

A small, soil-dwelling predatory mite, Hypoaspis miles, feeds on fungus gnat larvae. It is shipped in a vermiculite/peat carrier with all stages of the predatory mites. The vermiculite/peat carrier can be distributed over the media surface, especially when pots are placed close together.  These predatory mites are best used when fungus gnat populations are low and are compatible with Vectobac® and S. feltiae. 

Atheta coriaria
is a small black beetle called the Rove Beetle.  The beetle spends most of its life in the soil feeding on a variety of organisms including the eggs and larvae of fungus gnats and shoreflies, and pupae of thrips. All stages are very mobile in the soil, moving rapidly in search of prey. The adult beetle can fly and readily spreads throughout the greenhouse.

Chemical Control

A class of chemistry known as Insect Growth Regulators (IGR’s) are most effective against the young developing larvae and will have no direct activity against adults. Commercially available IGR's include Dimilin® and Citation®.  Both Dimilin® and Citation® are chitin synthesis inhibitors.  These products disrupt the creation of chitin, which is the main building block of the exoskeleton.  Precise timing is not required since insects molt for a long period of time and several times per lifecycle.  The newly formed endo-cuticle is distorted and cannot withstand the internal pressure during ecdysis and easily ruptures. 

Do not apply Dimilin® to poinsettias, hibiscus and Reiger Begonia.

For more general information about THE LOUPE, contact




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