Growers Guide: How to Prevent and Kill Fungus Gnats

Fungus gnats may not be the most prevalent pest cannabis cultivators face, but if they hit a grow during the early stages, these tiny flies can mean disastrous effects. A swarm of hungry fungus gnats can have serious implications for young seedlings and immature plants. With a short lifecycle, it's essential to manage fungus gnats early. 

Unlike other common pests, these small pesky flies don't nibble on leaves or suck nutrients from the foliage. Instead, they live in the soil and feed on the root system. As insects that live in soil, they are predominantly a concern for soil-based growers, but a dirty hydroponics operation can also see them fester.

As a grower, you'll need to know about fungus gnats and the signs of an infestation. If your cannabis seedlings and young plants develop an infestation, you'll also need to know how to prevent an infestation and kill fungus gnats if they do occur. So, this is your grower's guide to dealing with a little-known pest: the fungus gnat.

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Fungus gnats are small enough that you' might not notice them in the early stages of an infestation. According to the Farmers Almanac, they are roughly the size of a fruit fly and lay their eggs on the soil's surface. They prefer damp, moisture environments, which is typical within indoor soil-based cannabis operations. Hydroponics growers with a build-up of algae or fallen organic matter may also be susceptible to fungus gnats.

Adult fungus gnats lay upwards of 200 eggs on the surface of the soil. The eggs hatch in approximately three days into larvae, which seeks out food. Fungus gnat larvae feed exclusively on roots, and the larvae will burrow into the soil to find it. The life cycle completes when adult gnats emerge roughly 14 days later. 

Although these tiny insects are not harmful or dangerous to humans, they can damage young plants. Because fungus gnats feed on root systems, it prevents the uptake of water and nutrients. Mature cannabis plants, with robust root development, will likely survive with minimal stress. However, seedlings may die if the infestation is large enough.

• Small black flies jumping or swarming on the surface of the soil
• Yellowing leaves and spotting
• The appearance of nutrient deficiencies (see Potassium, Magnesium, and Calcium) 
• Stunted or sickly appearance
• Seedling die-off

Prevention of a marijuana fungus gnat infestation comes down to two things: moisture and cleanliness. Fungus gnats are primarily attracted to wet soil. If the soil's surface in your containers is constantly wet, you may be overwatering your plants anyway. 

If you don't have a Digital Moisture Meter or an automatic irrigation system, the standard practice is to water only once the soil's surface has dried out. Insert your finger near the base of the plant to check if it's time to water again. If the first two inches of the soil is dry, it is time to water.

The other preventative measure, which is especially true for hydroponic setups, is cleanliness. The more decaying organic matter (leaves, algae, etc.) exists, the more engaging the environment is for the fungus gnat. Remove all dead plant material, clean reservoirs, lines, and drainage holes of any organic build-up.

If you're growing in pots, clean the trays and any runoff catchment containers. Remember, the cleaner the facility, the fewer spaces for adult fungus gnats to lay eggs.

What should you do if you see a flurry of tiny bugs on the soil of your newly potted seedlings or clones? There are four safe ways to tamp out fungal gnat infestation, without damaging the bud with harsh chemicals.

1. Sticky fly paper: Sticky yellow flypaper attracts adult fungus gnats looking for a place to land. This prevents mature flies from mating and laying eggs. Place strips of yellow flypaper on the soil's surface, or hang from a lower branch. Traps are affordable and sold at garden supply stores. 
2. Apple Cider Vinegar: Apple cider vinegar attracts, traps and kills mature fungus gnats, much like fruit flies. Pour a small amount of apple cider vinegar in a shallow dish with a drop or two of dish soap. Set around the base of the plants.

3. Neem Oil: Neem oil is a tried and true natural pesticide often used within cannabis cultivation. Spray the surface of your soil with neem oil to kill larvae and eggs. This technique also works as a preventive measure. Try to catch them early, as neem is not recommended during flower as it can affect the flavor. 
4. Air Circulation: It's important to make the general environment less hospitable for fungus gnats in general. Set up fans to circulate air through the lower foliage and across the surface of the soil. While this won't eliminate the gnats, it a useful additional tool.
5. Diatomaceous Earth (DE): Usually used in conjunction with yellow sticky traps, this product is applied on top of soil and as the insects crawl through the DE, their exoskeleton is cut and they dehydrate. 

While fungus gnats are not the most destructive pest in the cannabis garden, they are not desirable. Especially for young plants, you'll want to prevent a fungus gnat infestation by keeping the space clean and avoiding the risks of overwatering. Catch them early enough, and there will be no danger to your plants.