Biological Fungus Gnat Control

Bacillus thuringiensis subsp. israelensis


P.O. Box 1555, Ventura, CA 93002

800-248-2847 * 805-643-5407 * fax 805-643-6267






Bacillus thuringiensis subspecies israelensis, the active ingredient in Gnatrol, is a common bacterium found naturally in soil. Bt is considered a “microbial insecticide” because several strains infect and kill insects. Since it was discovered in 1911, Bt has been developed for insect control, and was first available commercially in the 1950s.


Other strains of Bacillus thuringiensis bacteria have been discovered that target certain types of fly larvae (Bti, or israelensis strains). Insecticidal activity is specific among strains of Bt; strains developed for caterpillars do not affect fly larvae.



Bacillus thuringiensis bacteria degrade in sunlight. Most formulations persist on foliage less than a week following application. Some formulations become ineffective after 24 hours; check different formulations for information on effectiveness and persistence.


Bt must be ingested to be effective; therefore, application must be thorough. Sticking or wetting agents can be added to Bt applications to enhance coverage, resist washing, and improve performance.

Pest identification is important to distinguish fungus gnats from shore flies that are not affected by Gnatrol. Shore fly adults are larger with light-colored spots on dark wings. They are stronger fliers than fungus gnats.



The specificity of the bacteria works as an advantage in biological control programs because without the broad-spectrum activity, it is not harmful to beneficial insects and therefore integrates well with other natural control programs. The greatest advantage of Bt is that it is essentially nontoxic to humans, pets, and wildlife.


Many formulations are OMRI listed and can be used up until harvest on a wide variety of crops. The high level of safety of Bacillus thuringiensis makes it safe to work with in situations where exposure to pesticides during mixing and application are likely.


Gnatrol is a very specific pesticide that kills flies in the soil, its main use is for fungus gnat control. This product targets certain types of fly larvae including drain flies, black flies, fungus gnats, and mosquitoes. Gnatrol must be eaten by the susceptible insects. Once ingested, the Bacillus thuringiensis bacteria act by producing proteins that react with the gut lining of susceptible insects. These proteins, called delta-endotoxins, paralyze the digestive tract of the infected insect, which causes it to stop feeding within hours. The infected insect dies from starvation. On occasion, the bacteria will enter an insect’s bloodstream and reproduce, although, the most common cause of death is due to the reaction between the proteins and the digestive tract.


Gnatrol is a microbial insecticide that contains the naturally occurring bacterium Bacillus thuringiensis subspecies israelensis. The liquid contains 600 ITU per milligram (International Toxic Units). The dry granule (OMRI listed) contains 3,000 ITU per milligram. A rate of 2.2 to 4.4 billion ITU per 100 gallons is used for mild problems and increasing concentration for heavy infestations.







Gnatrol is a larvicide; it will not control adult flies and gnats. It is most effective when applications are timed when larvae are present in the soil. Monitor maggots by direct observation or place slices of potato snuggled into the soil surface, and check after 1-2 days. Monitor adults with yellow sticky traps horizontal just above the soil surface. For monitoring, some growers tolerate 5-10 per vertical sticky card per week especially in an integrated program where Hypoaspis or Atheta are present. Above that threshold, fungus gnat larvae are quickly brought down, responding to Gnatrol treatments within 24 hours. Repeated applications may be needed for the next generation of larvae.



For light infestations mix 16-32 oz Gnatrol per 100 gallons water (= 1 to 2 tsp/gallon water) and apply as a soil drench.


For heavy infestations mix 64-128 oz Gnatrol per 100 gallons water (= 4 to 8 tsp/gallon water) and apply as a soil drench.



Light infestation mix: 3.2 to 6.4 oz per 100 gallons

(= 0.9 g to 1.8 g per gallon or about ¼  to 1/3 tsp/gal)


Heavy infestation mix: 13 to 26 oz per 100 gallons

(= 3.7 g to 7.4 g per gallon or about 2/3 to 1.5 tsp/gal)


Note: Gnatrol WDG comes in a 16-pound pail.

16 lbs makes 10 gal and 1.6 lbs makes 1 gal of liquid Gnatrol


Dositron use

1:100 use 23 oz/gallon

1:50 use 23 oz/2 gallon


For 6-inch pots consider wetting the top inch of soil, using 1.5 to 2.0 oz diluted liquid per pot.


Apply Gnatrol as a soil drench, with sufficient water to thoroughly wet the soil surface above and below benches where larvae are present. Areas under benches should be treated at high rates. Reapply as needed if pest populations persist.


In existing infestations where all life stages are present, repeat applications weekly for three consecutive weeks at the rate for heavy infestations. Subsequent bi-weekly, or monthly applications at the light infestation rate will maintain control.



Gnatrol should be stored in a cool, dry place between 59ºF – 86ºF (15ºC – 30ºC). Liquid formulation has a shelf life of about 1 year and granular 2 years. Expect reduced effectiveness after prolonged storage of Bt-based products.



Gnatrol suspends readily in water and will remain suspended over normal application periods. Agitate after sitting for 15 minutes. Avoid continuous agitation of spray mixture during spraying. Brief re-circulation may be necessary if the spray mixture has been sitting for several hours. 



For success in an organic or biological program, moderate watering, adjust soil medium, exclude adult fungus gnats, and control pests in propagation. Hypoaspis, a predatory soil mite, or Atheta, a rove beetle, can be used early in the crop cycle for prevention because it can survive without fungus gnat larvae. Hypoaspis will not be harmed if fungus gnats exceed 20 per card indicating the need for Gnatrol treatments.


Steinernema feltiae, a beneficial insect parasitic nematode, is another a biocontrol for fungus gnats that eats a wider range of soilborne larvae and pupae. When thrips larvae are also present, start with Hypoaspis early and apply nematodes or Gnatrol as needed. Hypoaspis may eat some of the S. feltiae nematodes. Distance IGR by Valent is a low risk insecticide that can be co-applied with Gnatrol to get longer control. 



Gnatrol should not be used in combination with fertilizers or fungicides containing copper or chlorine, as this may neutralize the active ingredients. Chlorine levels in potable (city) water supplies should not present a problem with Gnatrol performance.


For best results, apply drenches toward the end of the irrigation period.


Do not apply soil drenches to plants under stress, or follow applications of Gnatrol with excessive amounts of water.


Read and follow the label instructions before using.

See MSDS sheet and product label for complete safety and first aid precautions.


ã Rincon-Vitova Insectaries, Inc. 2008