Fungus Gnats or Root Gnats are two common names for a few flying insects Diptera Mycetophilidae, Lycoriella spp. or Bradysia spp. These are arthropods that might be a nuisance flying around haphazardly knocking into leaves and the sides of your pots. I use organic substrates and I hate those critters. Growers using traditional hydroponic (using no substrate inoculates) see them a lot less. The adult fly is feasting on small pieces of organic matter breaking it down for even smaller organisms to mineralize. I hate gnats because once they have moved in, they are hard to move out. The adults are in-your-face but upon closer inspection you will find their larvae in the first few centimeters of your substrate. The larvae are so small you can barely see them with the naked eye. They feed on fungus for their livelihood. They also can get aggressive and do damage to some small root hairs as well.  It’s a dog eat dog, arthropod eat fungus world down their. Just be thankful your not part of the food chain.

Identifying Fungus Gnats

fungus-gnat-larveFungus-knat-Lycoriella-or-BradysiaFungus Gnats are darker and appear delicate, similar in appearance to small mosquitoes. You will first see the adults scampering around bottom leaves, the surface of the soil or the edge of your container. Females lay small, unnoticeable eggs in moist, organic potting soil. The adults are weak fliers often stumbling up on a leaf’s surface. If the adults are present you can count on having the larva down under, perhaps even eating your tips. The small squares in the image of gnat larvae to the right, are one mm square.

Controlling Fungus Gnats Organically

  • Prevention is the best method for the control of pesky pests
  • Inoculate the substrate with Bacillus thuringiensis
  • Place a barrier of some sort
  • Reenforce the barrier with an organic deterrent
  • Once infested place sticky glue on surfaces
  • Ten days latter… NO more gnats!

Controlling Fungus Gnats Organically, believe it or not, is a snap!

For prevention I put a bacteria to work right away upon planting. It takes a while after any microbial inoculation for the microbes to dig-in and reproduce, so upon planting, before gnats appear, is a good time to inoculate. The biological predator Bacillus thuringiensis subspecies israelensis, commonly called BTI is toxic to the larvae. Researchers investigated how this bacteria kills particular insects and discovered that BTI has two classes of toxins; cytolysins (Cyt) and crystal delta-endotoxins (Cyt)[1]. Cyt proteins are toxic towards Diptera. As a toxic mechanism, Cyt proteins bind to specific receptors on the membranes of mid-gut cells resulting in rupture of those cells[2].

Another preventative, of which I highly recommend, is to discourage the adult fungus gnats from laying eggs with a barrier to its nesting grounds, the substrate.  I have seen people place a half centimeter layer of clean sand on the substrates surface. However, I place a solid poly-barrier between the riotous mob and our organic substrate. The CO2 from the roots metabolism can escape but there are few holes a gnat can enter.It’s easy and an inherent part of our system.

Around the hole from which the plant stem protrudes from, place a deterrent like neem oil or/and pyrethrins. These will not decay and create more fungus food for the fungus gnats to eat. I was told there are two types of hot peppers. One is very hot, the other BURNS. I use the one that burns like fire, but the gnats barely noticed. Two days later the accumulation of a few spraying of hot pepper must of produced fungus. It was being eaten by the gnats. So don’t bother with organic compounds for the prevention nor eradication of gnats.

fungus-gnats-dead-48-hoursfungus-gnats-dead-12-hoursAn additional approach to deal with fungus gnat adults, particularly when populations are abundant, is to strategically place yellow sticky cards underneath the plant canopy or on the edge of containers. The adults are attracted to yellow and will be captured on the sticky cards. This may be helpful in mass-trapping adult females, thus reducing the number of larvae in the next generation. In practice, the round plastic disc I use as a barrier to the substrate, is a perfect place to apply the sticky glue. It is white yet very effective for trapping gnats. The images you see to the left & right, you can click on to enlarge. Get ready to witness the gnat graveyard. Its disgusting for many but a joy to look at for me. This image was taken only 12 hours after applying the glue. You can place glue on every fifth or sixth container top. It will take a weak or more for all the larvae to pupate. But afterwards NO MORE GNATS! But don’t get cocky. From one morning to the next… they could return. Keep applying the BTI.

 Controlling Fungus Gnats With Chemicals

Shame on you if you use chemicals and don’t think it will be more effective than glue. The most effective chemicals are those that are persistent, killing adult gnats for up to three days. A number of pyrethroid-based insecticides, with extended persistence, are available for use on plants including those containing the following active ingredients: bifenthrin, cyfluthrin, permethrin, and lambdacyhalothrin.

But remember… the larvae in the growing medium will not be directly affected by any insecticides applied to kill adults. Larvae may be killed by the microbial insecticide Bacillus thuringiensis subsp. israelensis (BTI) when applied as a drench to the growing medium.

References

1. Crickmore, N. et al. Revision of the nomenclature for the Bacillus thuringiensis pesticidal crystal proteins. Microbiol Mol Biol Rev 62, 807-813 (1998).

4. Dorsch, J.A. et al. Cry1a Toxins of Bacillus Thuringiensis Bind Specifically to a Region Adjacent to the Membrane-Proximal Extracellular Domain of Bt-R-1 in Manduca Sexta: Involvement of a Cadherin in the Entomopathogenicity of Bacillus Thuringiensis. Insect Biochemistry and Molecular Biology 32, 1025-1036 (2002).