Fungus Gnats - Bradysia spp.

Curtis Swift, Ph.D., Colorado State University Extension

Introduction:

Fungus gnats are small flies that are often seen in the fall and winter months.  Cool soil and over-watering contribute to their increase.  Soils with high organic contents have more problems with this pest. In addition to the adult flies being a nuisance, they spread Fusarium, Verticillium and Thielaviopsis on their bodies. Although these fungi are soil-inhabiting, they also sporulate on aboveground portions of their plant hosts. These hyphal swellings, microsclerotia and/or chlamydospores can then be picked up on the hairs and other body parts of the fungus gnat and transmitted to other plants.

Fungus gnat larvae (maggots) cause problems by feeding on plant roots causing wounds which contribute to root rot.  As they feed they ingest root-rotting pathogens which pass through the development stages of pupa to adult ensuring the adult has the pathogen in its gut when it emerges from the pupa. These pathogens are deposited in the feces of the adult. 

Why should you be concerned?

If you have houseplants or are planning on starting vegetable or flower seedlings you should be concerned about fungus gnats.  Pythium and Phytopthora are two of the fungal-like plant root pathogens transmitted by fungus gnats via their feces.  If you have had a seedling turn black and shrivel just above the soil line, it most likely was due to Pythium. Phytophthora generally attacks older plants causing the same problem. This condition is called damping off.  Placing a thin layer of vermiculite over the soil helps reduce problems with damping off at the soil line but does not eliminate below surface damping off problems. When the latter occurs the grower is more likely to blame the quality of the seed for the problem when a plant pathogen is the real culprit. If you have roots on your seedlings or mature plants that turned brown or black and died, it most likely was due to Phytophthora.  

Control:

Fungus gnat infestations in established plants can be eliminated with a bio-pesticide containing Bacillus thuringiensis sub species israelensis.  There are several different subspecies (strains) of B.t., each specific to certain types of insects.  The B.t. with israelensis is the one you need for fungus gnats.  Mixing this liquid with water and drenching the soil will eliminate the maggots.  To keep fungus gnats out of your plants in the first place, get in the habit of pasteurizing the soil you use. This is especially critical if you are starting seedlings.

Pasteuring soil in the microwave oven

Fungus gnat eggs and maggots as well as Pythium and Phytophthora are best eliminated in potting soil by pasteurization.  Greenhouses typically pasteurize huge batches of soil with steam.  You can do the same thing in your oven.   It is smelly but will help eliminate problems with damping off and rid the soil of fungus gnats.

The soil needs to be moist for heat transfer.  Place the soil in an oven-safe container and put it in the oven. When the temperature of the soil reaches 180 degrees Fahrenheit maintain that temperature for 30 minutes.  Let the soil cool and it is ready to use.  Avoid the tendency to cook the soil at a higher temperature or for longer than 30 minutes as this can result in the release of toxic volatiles.  I find a meat thermometer inserted into the center of the soil mass or using a temperature probe that plugs into your oven is the best way to determine when the soil is ‘done’.  If you would prefer to use your microwave it will take 90 seconds at full power for every 2 pounds of moist soil.  Once the soil has been pasteurized keep it in a sealed plastic bag or some other air-tight container until it is used.  This will prevent recontamination by fungus gnats and plant pathogens.

Reference Used:

Hyder, N., Coffey, M.D., and Stanghellini, M.E. 2009. Viability of oomycete propagules following ingestion and excretion by fungus gnats, shore flies, and snails. Plant Dis. 93:720-726.
Kalb, D.W., and Millar, R.L. 1986. Dispersal of Verticillium albo-atrum by the fungus tnat (Bradysia impatiens). Plant Dis. 70:752-753.

Additional Information on Fungus Gnats:
http://www.ext.colostate.edu/PUBS/INSECT/05584.html

Placed on the Internet January 25, 2010