Managing Fungus gnats and protecting young plants

About fungus gnats and why they thrive in our homes

Fungus gnats (Bradysia spp.) are among the most common house plant pests. Many of the indoor house plants we keep are tropical in nature. They are from the forest floor layer. Conditions for house plants and seedlings are outstanding conditions for fungus gnats. They are a vector for diseases and damage. 

Myths of controlling fungus gnats

Watering

Overwatering will lead to fungus gnats. Typically regular watering is enough to bring in fungus gnats. Even a reduction of water will not kill off the larva. I have seen soil completely dry with living fungus gnat larvae. When conditions are like this, the larva will keep itself in a single location within the soil and wait for the next watering. Unfortunately, keeping most plants in these conditions for too long will kill off most plants in the process. This does deter adults from laying eggs in the media. The lack of water might kill off some of the eggs if you are trying to reduce the number of fungus gnats in your home or grow area. You must treat the entire area as a problem, as they will hop between plants. This control method is best if you can move things outdoors so the pest can find a more suitable location for the life cycle. 

Coco Coir is a better media than peat moss for fungus gnats

Data does not support this claim. Some products claim that the fine particles in coco coir reduce the number of pests. The data shows no supporting evidence for this claim. The marginal differences between the two media show no significant differences. Sterilized, lack of food, and with food showed that all media were virtually the same. 

Neem is an effective control.

Neem does not reduce fungus gnats. Most active ingredients are lost in neem unless it is extracted correctly. Cold press products tend to have active ingredients, but marking and deceptive practices have made it hard to determine what is a good product. True neem products can significantly reduce the number of larvae.

Controlling and eliminating fungus gnats

Mechanical controls 

Yellow sticky paper is an excellent way to spot fungus gnats early. Fungus gnats are attracted to the color and allow for easy identification. As they are a little larger than fruit flies, you might be able to make them out with your naked eye. A small magnifying glass may help with identification. Early detection is best with any pest problem. The yellow fly paper also helps keep their population within manageable levels. Early detection is key for any pest problem. 

LEDs with the yellow pigment can be used to draw in gnats and other pets as a control. With sticky paper around the LED. 530nm lime green was used in a study to control multiple pests species and was shown to be the most effective. 

Diatomaceous earth reduces the number of fungus gnats emerging from the soil. The water content of the media does play a role in DE. I have yet to see much results with this when in use. I only used it as a top dressing, and it clumped together, and I would see fungus gnats go right through the caked layer of DE after it had dried and hardened. It might be best to mix it into the media. 

Sand can be used but reduces the airflow in your media. Gnats will still crawl in drain ports on the bottom of the pot and will not protect your plants. Doing a layer on the bottom and top of your soil is possible. Ensure you have a soil mix that promotes the transfer of air, or you could end up with other diseases. Sand isn’t a standard method, but it does work.

Biological controls

Bacillus thuringiensis israelensis (Bti) is a bacteria that gets lodged in the larva’s gut and stops it from eating. This needs to be periodically applied after signs of an infestation. Only the israelensis subspecies is effective against fungus gnats. It should be applied as a soil drench. So finding a Bti source in liquid form is preferred. Bti bites that you often see in stores may not be practical due to their low concentration needed to kill mosquito larvae. The Bti bites and dunks can still be used. I have heard people getting good results shaking them up in water bottles and other applicators to use the Bti. 

Nematodes are fast-acting and effective against fungus gnat larvae. The issues I have had with these products are dead on arrival. I have attempted to source this from 3 different sources, and each time… They were all dead. I used distilled water and a glass slide to view them under a microscope. I had no movement, but I had nematodes. Unfortunately, they need to be alive to be an effective control. 

Stratiolaelaps scimitus (Hypoaspis miles) feed on both the larva and eggs. I have had the most success receiving live mites over live nematodes. They feed on other insects, and if you have an ecosystem. They can survive much longer within your grow area. They are also compatible to be used with nematodes. This would be for a more aggressive biological control of fungus gnats. The mites are only active on the top 2-3 inches. This is enough to control most fungus gnats. 

Chemical controls

Neem (Azadirachtin) works by causing deformations during the molting stage and can slowly kill the larva. Follow the manufacturer’s recommendations. The best control would be a soil drench, per the manufacturer. It needs to be a cold-pressed product; otherwise, it will not likely contain the active ingredient in neem. Not all neem products are created equal. Neem dramatically reduces the number of adult fungus gnats that emerge from the soil. It would be best if you looked for Azadirachtin on the label of the active ingredients of your neem product. Some sources I have read state that neem is unsuitable for a long-term solution for fungus gnats.

Pyrethrin & Pyrethroids are fast-acting pesticides that are highly effective against the adult fungus gnat. This has a drop-dead effect and is nearly instant in most cases. Also, these products lack stability in the environment and tend to last only a short time after application. Refer to the label for proper use. They tend to lack effect on eggs and larvae. 

Insect growth regulator (IGR) is an effective control for lavra, as it stops the proper growth of larvae and pupa. These are slower but broad-spectrum depending on the product. 

Hydrogen Peroxide(H202) is commonly recommended on youtube. This does work but is harmful to the soil’s ecosystem and the plants. Using too much will damage the plant and the plant’s roots. Peroxide is very volatile and rapidly reacts to organic matter. This would require a large amount of peroxide to be effective in the soil. 

After all, why do we care so much about fungus gnats in the garden when starting seedlings and with crop yields?

Fungus gnats are a vector of diseases of plants, especially young seedlings. Within 2 hours of a fungus gnat arriving, it can have had spores that were carrying on the plant’s body infect host plants. This results in seedling death after it is infected and is a factor in damping off. As these fungi thrive in the humid conditions that we grow seeds in. Seedling growth can be stunted by eating their young roots and reducing yield in adult plants. I have had slower-growing plants, such as cacti seedlings, fall over due to their roots being eaten.