Fruit Flies or Fungus Gnats on a Lemon Tree: Why you have them and how to get rid of them

A couple months ago I went to water my lemon tree and upon doing so found several buzzing, annoying tiny creatures flying out of the soil. “Oh no, fruit flies…” I thought with a sigh. I ignored them for several more weeks hoping they would go away without me doing anything. They didn’t, of course. They got worse. So, I took to research to figure out what was going on and how to get rid of them.

To my surprise, I learned the insects pestering me lemon tree were not fruit flies but fungus gnats!

While fruit flies and fungus gnats look similar, they do have discernable visual differences. But what you’re dealing with can be most easily determined by where the insects are congregating. If they’re in your kitchen eyeing up the ripening produce, they’re fruit flies. If they’re flying around and burrowing into the soil of your lemon tree, they’re fungus gnats.

Regardless, you don’t want an infestation of fungus gnats for several reasons. Read on to find out what I learned about why your lemon tree could be fostering an infestation, and what you can do to eliminate the problem.

Difference Between Fruit Flies and Fungus Gnats

Fruit flies and fungus gnats can look pretty similar if you don’t know what you’re looking for.

Fruit flies can range from tan to black in color, but have large red eyes that can give their appearance a reddish hue. Fruit flies are rounder and more closely resemble a very miniature house fly in shape.

Fungus gnats are dark grey and slimmer. They appear lankier, and look more like miniature or less exaggerated mosquito.

The biggest indicator, however, is the location of the insects. If your pests are hanging out in the kitchen with your produce, they’re fruit flies. If they’re hopping around and burrowing in the soil of your houseplant, they’re fungus gnats.

Do Fungus Gnats Pose a Health Danger to Lemon Trees?

A few fungus gnats won’t cause any damage to your lemon tree and are more of a nuisance. However, it is easy for a few fungus gnats to turn into a full-blown infestation if conditions are hospitable and corrective measures are not taken.

Adult fungus gnats are relatively harmless, aside from their ability to lay eggs. Adult fungus gnats do not feed on plants, do not bite humans or animals, and aren’t carriers of diseases. It is the fungus gnat larvae that pose the greatest danger to your lemon tree. Larvae lurk underneath the surface of the soil and feed off of decomposing organic matter in the soil. When the larvae run out of other organic compounds in the soil to feed off of, they will turn to feeding off of your lemon tree’s roots.

Although rare, a severe infestation of fungus gnats that is not taken care of and left to fester and reproduce exponentially over time, could ultimately result in the death of your lemon tree. Fortunately, there are many, easy solutions you can apply to remedy conditions and eliminate your fungus gnat problem in no time.

Why Your Lemon Tree Has Fungus Gnats

It is possible for fungus gnats can come in from the outdoors, but it is most likely that they were already present as eggs in a new plant or a bag of soil you purchased. They are sneaky creatures, though, and can seem to appear out of nowhere. What attracts them and causes them to stay and reproduce, however, is more of a concern and controllable variable than their origin.

Fungus gnats thrive in soil where organic matter is decomposing due to a consistently moist state. Consequentially, fungus gnats are usually a sign that you are overwatering your lemon tree. If you are watering your lemon tree too frequently and the soil is never left to dry out between waterings, conditions will become ripe for a fungus gnat infestation.

What You Can Do to Get Rid of Fungus Gnats

  1. Reduce the frequency with which you water your lemon tree. Fungus gnats are a sign you are overwatering your lemon tree. If soil is left for enough time to dry out between waterings, this becomes an inhospitable environment for fungus gnats and they will not be able to reproduce and become a problem. Note: There are additional dangers to overwatering your lemon tree, such as root rot or fungal infection, so take care to remedy this immediately, yet without overcorrecting.
  2. Discard the top inch of your soil and replace with gravel or sand. By discarding the top layer of soil, you will hopefully be disposing of any fungus gnat eggs that have not hatched. And because sand and gravel are free of organic matter and dry out quickly, by layering either of these materials on top of your lemon tree’s soil, you will be discouraging the laying of any new eggs, and therefore putting an end to the reproductive cycle of your fungus gnats.
  3. Set traps. There are a variety of trap situations which you can implement to attract and kill adult fungus gnats. I’ve had personal success with making using sticky traps. It is easy to make your own. Take an index card and color it bright yellow with a highlighter or marker, or find a brightly colored piece of cardboard from a household package. Adhere the index card or cardboard piece to a popsicle stick or equivalent. Then cover the index card or cardboard with petroleum jelly. The gnats will be attracted to your brightly colored trap and will fly into it. They will then become stuck by the petroleum jelly and will be unable to fly away. You can also skip the fuss and buy professionally made sticky traps here.
  4. Generously mix diatomaceous earth with the top inch of your lemon tree’s soil. Mixing diatomaceous earth with the top of your lemon tree soil can be a good option for fungus gnat control. Diatomaceous earth is comprised of tiny little razor-sharp particle of silica. When larvae turn into adult gnats and crawl out of the soil, the diatomaceous earth will cut through the gnat’s exoskeleton and absorb its moisture, causing the gnat to dehydrate and die. Diatomaceous earth is organic, and also a fairly common household product that has many applications for pest control, so you might already have laying around the house.
  5. As a last resort, mix hydrogen peroxide with your next watering. If your gnat infestation is severe or if you are not having any luck with other, gentler remedies, consider using hydrogen peroxide. Mix one-part 3% hydrogen peroxide with four parts water. Fungus gnat larvae will die upon contact with hydrogen peroxide. Don’t worry, on occasion, a dose of diluted hydrogen peroxide will not be harmful to your lemon tree. However, refrain from using any higher concentration of hydrogen peroxide than 1 to 4, as it could cause root burn.

How to Prevent a Fungus Gnat Infestation of Your Lemon Tree

  1. Use a well-draining soil mix formulated for citrus. When you pot or repot your lemon tree, the mix you choose should contain a heavy portion of sand. Also consider mixing in some vermiculite or perlite, which will help with drainage and aeration.
  2. Choose a pot for your lemon that is made from a material that breathes. Plastic or glazed pots are non-porous, and trap water by the barrier of their walls. Choosing a terra-cotta or unglazed pot will allow water to be absorbed by and pass through the walls of the pot, making it less likely that your lemon tree’s soil will remain too excessively waterlogged.
  3. Refrain from using wood chips, mulch, or compost. If you’re also an outdoor gardener, this advice might seem counterintuitive. While wood ships, mulch, and compost work wonders for outdoor plants, they are unnecessary for potted plants, and are composed or organic matter that fungus gnats love and thrive off of. Because of this, it is best to steer away from these additives for indoor plants, altogether.
  4. Develop an appropriate watering schedule. Stick your finger about two inches below the surface of the soil of your potted lemon tree. Is it dry, or still moist? If it is moist, it is not yet time to water. If it is dry to the touch, it’s time to give your lemon tree a deep watering. Sticking to this rule of thumb can help you avoid the dangers that both over and under-watering your lemon tree can bring.

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