SUMMARY of BugBites! Session 5: Fungus Gnats

Monday, June 20, 2022

Although fungus gnats (Sciaridae) are primarily seen as a nuisance in indoor production spaces, they can cause a lot more damage than the eye can see! As a result of their favourite food source (plant roots), these pests are capable of acquiring and spreading fungal disease amongst the crop.

The following information is a summary of session 5 of the BugBites Series: Fungus Gnats and Shorefly Suppression with Beneficial Organisms. To watch the full session, click here

Pest Description: 

Adult fungus gnats are mosquito-like insects with long legs, and a characteristic y-shaped wing vein. Adults can be found hanging around or near the soil, mostly under plant canopy. The fungus gnat larvae are white with a shiny black head. Larvae spend most of their time underground and will feed on the roots. 

Figure 1. A photo of an adult fungus gnat stuck to a yellow sticky card. Below, is an image of the fungus gnat larvae. If you look closely, you can see the characteristic black head at the end of the larva. Image taken from the BugBites! Session 5 presentation. 

Monitoring and Scouting: 

Adults are not strong fliers, meaning that sticky cards will only be effective when placed close to the soil line. If placed correctly, traditional sticky cards are a great tool to monitor population sizes.

As an attractant, some growers will use half a potato, or a potato slice, placed in a pot to draw larvae up from the soil. As the larvae are accustomed to feeding on roots, the use of a tuber is a big draw.

For the adults there are various traps available. These traps typically involve the use of an attractant (ex. UV bulb) and a way to capture the insects (sticky cards, soapy water solution etc.). This is useful for general scouting purposes as well as mass trapping.

Biological Control Options: 

As with any biological control program, prevention is the first line of defense in controlling population sizes. By conducting regular scouting checks of the crop, and maintaining populations of biological control agents, you can limit the degree of pest infestation.

Dalotia coriaria; Rove beetle

  •  Predator to both shoreflies and fungus gnats
  •  Both adults and larvae are predators
    • Orange larvae will be noticeable in your soil
    • Adults readily move around via flight
  • Establish readily in greenhouses and work well in hydroponic systems

Figure 2. A photo of a rove beetle in the adult (left) and larval (right) stages. stuck to a yellow sticky card.  Image taken from the BugBites! Session 5 presentation

The use of rearing boxes allows for a slow-release approach and will help maintain beetle populations through the season. Rearing boxes can be as basic as media (ex. peat, vermiculite) set inside a Tupperware container, and supplemented with food (ex dog or fish food, artemia). It is crucial to keep this moist, but not wet. These can be placed under benches.

Bacillus thuringeniensis israeleinsis 

  •  Functions as a stomach toxin for fungus gnats
  •  Best applied via soil drench or chemigation. Irrigation is not ideal as the soil needs to get very wet to allow for good establishment of the biopesticide

Stratiolaelaps scimitus; predatory mite

  • Target fungus gnat larvae
  •  Well suited for greenhouse environments: optimum development temperature of 25oC (77oF)
    • Ideal range is 15-30oC (59-86oF)
  • Live in the top half inch of the soil: best applied to the substrate surface after transplant
    • Want to avoid burying too deep in the growing media
  • Does not establish well in rockwool
  • Persistent control agents, and can provide season-long protection

Figure 3. An image of an adult predatory mite. Image taken from the BugBites! Session 5 presentation

S. scimitus can be mixed with Dalotia, but it is not recommended to introduce them before adding into the crop. They will feed on each other, so make sure they are spread evenly throughout the canopy when distributed.

Steinernema feltiae; entemopathogenic nematode

  • Target fungus gnats, shoreflies, thrips pupae and many others
  • Forage in the top inch or two of the soil
    • Water into your growing media lightly- if they get too far in, they will not work
  • Not highly mobile
  • Well suited for greenhouse environments with an ideal temperature range of 10-25oC (50-78oF)
  • Susceptible to being washed away in rockwool
    • When growing in rockwool, apply after the last irrigation of the day to give them a chance to establish

By ensuring thorough protection against all life stages of the fungus gnat, you should be able to keep populations at bay! Prevention is everything, so be sure to apply biologicals preventatively and as needed.

Posted by: Talia Plaskett