Gnats, in general, are nuisance pests that frustrate us by lurking around our faces often times resulting in a very strong urge to smack them, if by missing them we wouldn’t hurt ourselves.

Fungus gnats more specifically look like small mosquitoes and are seen hovering on plants and even more so on damp soil, sewers, drains and organic/decomposing substrates that collect moisture. The larvae of these gnats feed on roots and often noticed in conjunction with root rot and kill small plants. The adults, however, do not damage the plant itself. They search for the best locations to lay their eggs.

The most likely scenario that you encounter fungus gnats in your citrus growing experience is when there is excessive moisture in your potting mix in the case of a potted plant or in clayey soil where the water is not draining.
Before looking into what can kill a fungus gnat, it is important to address the root cause of their presence. Fungus gnats in itself are not a significant pest of citrus. By that, I mean they are not attracted to a citrus plant due to the tender new leaf growth, the flowers, or the fruit they produce. It is a pest of opportunity when moisture is abundant and supports its lifecycle. That means, if the soil is allowed to dry between watering cycles, the fungus gnat life cycle can be broken and they cannot thrive. Their entire life cycle takes about half a month depending on the weather and can have multiple overlapping generations when they thrive. Once they get going, their numbers can be overwhelming and hence controlling them is important.

Breaking their lifecycle or preventing fungus gnats in the first place should be the ideal course of action. Also important is the fact that Citrus doesn’t like wet feet. If you constantly overwater your citrus, that would lead to defoliation and killing your plant. Further, waterlogged situations also result in phytophthora infections which will also take your plant down eventually.

For some reason, if controlling the water is not possible immediately, we need to explore other options.

Home remedies:
1. Hydrogen Peroxide: one part hydrogen peroxide in 4 parts water is a good mixture that will kill the larvae. During application, you will hear fizzing sounds for a while. This is the peroxide reacting with and breaking down to Oxygen and water and is no reason for worry.
2. Sand: Helps drain water. So adding a layer of sand on top of existing moist soil does couple things.
a. Keeps the top layer dry and breaks the lifecycle of the gnat
b. Kills the larvae under and prevents any further development.
3. Sticky traps: using some Vaseline or oil on a yellow cardstock would help attract the adult gnats stick to the cards to varying degrees of success.

Biological methods:
1. Diatomaceous earth: it is basically fossil dust, hence natural. Since they contain silica particles, they rupture the cuticle when insects pass over them. Food grade diatomaceous earth is safer to handle, but nevertheless using a face mask to prevent inhalation is a good precaution to take.
2. Bacillus thuringenssis: a naturally occurring bacteria that infects the larvae and kills them. These are also available as formulations that can be mixed with water an applied to control several different larval insects.
3. Nematodes: natural enemies of these larvae, nematodes feed on them. Some of them are naturally available in the soil, and in other areas, they need to be applied. Several formulations of these are commercially available.
4. Beuaveria bassiana is a fungus which is a natural enemy of fungus gnats and will kill them. Formulations of these are also available through various vendors.

Suggested Reading:

Fungus gnats management guidelines

Fungus gnats as houseplants and indoor pests 

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