Keeping Fungus Gnats Under Control

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Fungus gnats: often more annoying than really harmful. Source: &

You’re sitting quietly in your living room and, suddenly, a tiny black fly zigzags in front of your face. You chase it off with a wave of the hand. Attracted by the CO2 in your breath, it soon comes back. You manage to squash it this time, but soon another one appears.

What is this insect? And where does it come from?

Many people mistake this tiny pest for a fruit fly (Drosophila spp.), and it is about the right size, but this particular little creature has no interest in fruits. Instead, it seems mostly to hang out around your firewood or your houseplants … that is, when it’s not flitting about your face.

The pest in question is a fungus gnat, a fly in the Sciaridae family of which there are some 20,000 species worldwide. Most of those that infest our houseplants are in the genera Scatella, Bradysia, Orfelia and Sciara, a rather useless bit of information, because it’s unlikely you’ll ever know the name of the exact species that shows up in your home.

A Family Portrait

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You’d need a magnifying lens to see this much detail on such a tiny insect as a fungus gnat. Source: Luis Miguel Bugallo Sánchez, Wikimedia Commons

The adult fungus gnat is a very small fly, rarely more than 4 mm in length, and usually black with transparent or grayish wings. You’ll often find several of them in the vicinity of a pile of firewood or around houseplants, but it also roams all over the house and, as mentioned, seems particularly interested in the air you breathe out.

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Fungus gnat larvae. Source:

The larva, which resembles a whitish, translucent, black-headed worm about 5 mm long, needs moisture to survive and will be found in some sort of moist organic medium, such as damp bark or potting soil.

How do fungus gnats get into your house? Well, adults may not be the world’s greatest flyers, but they can certainly be blown indoors or travel in on a pet or clinging to someone’s clothes. And it only takes one fertilized female to start a new horde!

That’s not the most common method, though. Most of the time, you (and yes, I’m pointing my finger in your direction!) brought eggs into the house by accident, either on firewood or when you bought a new houseplant or a bag of contaminated potting soil.

Most gardeners assume potting soils and seed starting mixes have been sterilized, but in fact, that’s almost never the case. Potting soil manufacturers instead use ingredients that are unlikely to harbor pests, but that do include some beneficial organisms: organic ingredients like peat, chopped bark and coir. They don’t want to sterilize their blends, as that would kill the beneficials. However, even if the potting mix is shipped out sans fungus gnat eggs, that doesn’t mean it will stay free of them.

When the bags of soil make it to the retail store, they often sit there for months and any tiny hole in the bag, even one no larger than a pin prick, is enough for female fungus gnats to lay their eggs in. Since garden centers inevitably host at least a small population of fungus gnats (they’re pretty much ubiquitous in humid environments), it’s not at all rare to bring home a potting or seed-starting mix already contaminated with fungus gnat eggs.

Good News, Bad News

The good news is that fungi are usually quite harmless.

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Adult fungus gnat. Source:

Other than annoying humans by their mere presence (who wants to see Aunt Mathilda swatting at gnats when she’s over for a visit?), adult fungus gnats cause no damage. Yes, they have mouth parts, but they neither bite nor sting. In fact, many even make themselves useful by pollinating flowers!

Even the larvae are usually not very harmful to plants. They mostly consume decaying plant matter they find in the soil mix as well as small fungi and algae. They’re especially attracted to the types of fungus found growing in overly moist soil. And here too, they can make themselves useful, because by digesting these products, they produce mineral-rich droppings plants can feed on.

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Some species of fungus gnat will attack plant roots if there is little else to eat. Source: Clipart Library &

On the other hand, especially when there is nothing else to eat, the larvae of some species (but not all) do attack plant roots. On a healthy plant, this is rarely very harmful, however. To start with, most houseplants produce more roots than they really need and in fact, a bit of fungus gnat pruning often stimulates the plant to develop a denser and more efficient root system! Plus, fungus gnats generally prefer dead or dying roots to live ones, especially tending to feed on roots that have been immersed in water too long and have started to rot. A mature plant has to be in pretty bad shape for fungus gnats to do any major damage.

Their effect on seedlings, on the other hand, can be much more serious. Young plants have very limited root systems and rather weak ones at that. In addition, adult flies can carry spores of harmful fungi, such as the dreaded seedling disease known as damping off. Plus, the small wounds fungus gnat larvae leave can serve as an open door for rot to set in. So, you really don’t want them chomping on the roots of your seedlings.

That’s why, even if the fungus gnats are not always very harmful, eliminating them is still the best policy … and if you discover they’ve found your seed trays, you need to get rid of them as soon as possible.

Know Thine Enemy

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Source: Knock-Out Gnats Granules

Indoors, fungus gnats can be present in any season and produce several to many generations a year. Although the adult seldom lives longer than 8 days, the female still has time to lay about 50 to 200 tiny eggs in moist potting soil or organic debris, preferably in the presence of fungi (it’s not for nothing they’re called fungus gnats: the odor of fungus attracts them). The eggs hatch in about 5 days and the larvae then feed for another 14 days. Next they pupate for 5 days. Following that, the adults emerge and the cycle begins again. The entire process takes about a month under normal indoor conditions, but can take place in as few as 17 days in a hot, humid greenhouse.

How to Control Fungus Gnats

Reduce Your Watering


Letting the potting mix dry out before you water can sometimes eliminate fungus gnats entirely. Source:

The first key to success in controlling fungus gnats in indoor plants is to understand that they absolutely need constantly moist potting soil during their larval stage. If you make a habit of letting your plants dry out a little longer, this will often solve the problem completely. In fact, many experts consider fungus gnats more a symptom of overwatering than a problem in itself.

Try applying the golden rule of watering. That is, water deeply, enough to moisten the entire root ball, then allow the soil to dry before watering again. Many gardeners discover that careful watering keeps fungus gnats fully under control.

Don’t be discouraged if this doesn’t seem to work at first: it takes at least a month to before the treatment will have reached all the stages of the insect’s life cycle.

A Barrier Between Soil and Air

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By covering the soil with sand or small stones (here: Growstone Gnat Nix top dressing, made from recycled glass), you can eliminate fungus gnats. Source: Growstone Gnat Nix

Another possibility is to make the surface soil unfit for fungus gnat larvae. Larvae live near the surface of the soil, no more than 1 ¼ inch (3 cm) deep, as they have to regularly move to the surface to breathe. Plus, they need moist, organic soil to live in. If you cover the soil with a ½ inch (1 cm) layer of sand or pebbles (there are even commercial products especially developed for this purpose, such as Gnat Nix), the larvae will no longer be able to live there … and besides, adults won’t be able to lay their eggs on such a dry medium either.

Trap the Adults


Light traps catch all flying indoor insects. Source:

To control adults, you can try setting out yellow sticky traps or, better yet, a light trap. I find the latter works like a charm!

Bring Out the Insecticides

It is also possible to drench the soil with an insecticide. Might I suggest one with few adverse effects on the environment, such as insecticidal soap or neem oil? It is also possible to make the soil less attractive to larvae by mixing diatomaceous earth or cinnamon powder into it.

Predators to the Rescue

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Knock-Out-Gnats brand Bti. Source:

The biological larvicide Bti* (Bacillus thuringensis israelensis), a bacterium usually used to manage mosquitoes, will help control fungus gnat larvae as well. Make a solution and use it to drench the potting soil. There are even formulations developed specifically for this purpose, such as Knock-Out-Gnats and Gnatrol, two professional products you can find on the Internet. You can also take MosquitoDunk type pellets, likely available locally, and crush them into powder to make a solution.

*Do not use Btk (Bacillus thuringensis kurstaki), designed for use on butterfly larvae, on fungus gnats. You need Bti, specific to Diptera larvae (flies).

Bti will only be effective when larvae are present in the soil. It has no effect against adults, eggs or pupae. Larvae, though, quickly consume the bacteria and die within 24 hours. It will likely be necessary to repeat the Bti treatment weekly for 3 or 4 weeks in order to eliminate all the pests as they cycle through the various stages of their life.

There are also other predators you can use to control fungus gnats, including beneficial nematodes like Steinernema feltiae or predatory mites like Hypoaspis miles. You can order these predators on the Internet.

Fungus gnats: often more annoying than really harmful and fairly easy to eliminate as well. Don’t lose too much sleep over them!20180111B Luis Miguel Bugallo Sánchez, WC


Keeping Mosquitoes Out Of Your Water Garden



With mosquito-borne diseases (West Nile virus, equine encephalitis, Zika* virus, etc.) becoming increasingly common and worries about sources of stagnant water hosting them in backyards on the rise, it’s nice to know that there is an easy way to make your water garden isn’t to blame.

*The Zika virus is not yet found in North America north of Florida and Texas and is entirely absent from Europe, but mosquito species capable of carrying this disease are present on both continents. Most authorities believe that it is only a matter of time before the virus reaches these areas.

It’s easy enough to control mosquito larvae (and black-fly larvae as well) and thus help cut back on the number of biting adults in your garden. You just have to add Bti (Bacillus thuringiensis israelensis), a beneficial bacterium, to the water.

No, Bti is not a GMO. It’s a naturally occurring, widely distributed bacterium probably already found in swamps and lakes near where you live. By releasing it into your backyard pond, you’re only making sure that it is protected as well. Bti is, of course, organic and is widely accepted in organic gardening.

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Bit will kill mosquito larvae before they turn into biting adults. Photo: Pixabay

Bti is specific to mosquito and black-fly larvae and will not harm any other insects. Not only is it safe with bees and butterflies, but won’t even harm other aquatic insects. (Please note that another form of the Bt, Btk [Bacillus thuringiensis kurstaki], is used for the biological control of caterpillars, that is, butterfly larvae.) Nor is it toxic or harmful to fish, birds, amphibians, reptiles, or mammals, including humans and domestic animals.


These Bti pellets look like floating Cherrios!

Bti is found in several formats: granules, powder, floating pellets, etc. Depending on the option chosen, treatments should be repeated approximately every 30 days.

Products containing Bti are found in hardware stores and agricultural coops as well as on the Internet (for example, on Here are a few of the trademarks offered: MosquitoDunks®, AquaBac and Vectobac.

Finally, here is a more in-depth source of information about Bti: Everything you should know about Bti.20170725A

Bt: The Garden-Friendly Bacteria


On vend du Btk pour le contrôle des chenilles.

When we think of bacteria, we always tend to conjure up the harmful varieties, such as flesh-eating bacteria (Streptococcus Group A), salmonella (Salmonella spp.) or E. coli (Escherichia coli). Few people think about beneficial bacteria and this, in spite of the fact that are actually many of them all around us.

The best-known beneficial bacteria in the gardening world is without any doubt Bt (for Bacillus thuringiensis), a bacteria very abundant in the wild. It is found just about everywhere, almost certainly in your own backyard. We know dozens of strains of this useful microbe, many of which are very specific to certain groups of insect hosts, hence of great interest to gardeners.


Bt causes its host to stop eating and die.

All work in much the same way: Bt remains dormant on the plant, in the soil or in water until it is ingested by its host insect. In the insect’s digestive tract, it starts to grow and release toxic crystals, making the insect sick. Soon (within three to four hours of ingestion in the case of caterpillars), the insect stops eating and eventually dies. Since this can take from a few hours to over a week, gardeners have to be patient: those who expect to see the pest die within minutes of the treatment will be sorely disappointed!

Note that Bt has no effect on humans, pets, plants, or wildlife… other than the specific categories of insects Mother Nature designed it to infest.

Several Bt strains are being offered as organic pesticides and probably even more strains will be in the future. In fact, Bt is presently the most widely used bio-insecticide in the world.

Knocking Out Caterpillars


Cabbage butterfly caterpillar. Photo: Wikimedia Commons

Currently, the most commonly available strain is Btk (Bacillus thuringiensis kurstaki), effective against butterfly and moth larvae: in other words, caterpillars. But be careful! Like any pesticide, even organic ones, it must be used carefully, strictly on infested plants (such as on crucifers to prevent the cabbage butterfly, for example, or on garlic and onions to control leek moth), because it will also kill the caterpillars of any butterfly or moth, even those considered harmless or even endangered. Also, you have to make sure that the pest you use it on truly is a caterpillar (that is, an insect from the Lepidoptera, the butterfly and moth family), not something else. Btk will not be effective, for example, against sawfly larvae, although they look like small caterpillars, because sawflies are actually in the Hymenoptera or wasp family.

Readers living near the coniferous forests of Northern North America may well know about Btk as it is the insecticide most commonly sprayed on forests to control spruce budworm (Choristoneura fumiferana).

Mosquitoes Get Blasted Too


Bti is often sold in the form of donut-shaped capsules designed to float on water.

Bti (Bacillus thuringiensis israelensis) is a strain of Bt that is effective against mosquito and blackfly larvae. Many municipalities use it to make life more pleasant for their citizens. And with all the press coverage these days about the Zika virus, a mosquito-transmitted disease, Bti is currently much discussed in the media. However, while mosquitoes and blackflies may annoy people (and yes, sometimes even carry diseases that make them sick), they nonetheless play an essential role in the environment. Many animals depend on mosquitos for their survival and that includes many desirable bird species as well as bats. For that reason, the decision on whether or not to use or Btk, especially on a large scale, will probably always remain controversial.

Down With Potato Bugs


Colorado potato beetle. Photo: Wikimedia Commons.

There is even a form of Bt that is used to control the Colorado potato beetle (Leptinotarsa decemlineata), also called potato bug: Btt (B. thuringiensis tenebrionis). This product has not proven as effective as the other strains and is not approved for domestic use in many countries. That’s because, firstly, results on potato beetles are sometimes disappointing, but also, it affects a wide range of beetles, some of which are beneficial. Of particular concern is the fact it is considered toxic (although minimally so) to honeybees.

Decisions, Decisions!

Although Bt is considered organic (of course, since it is a living bacteria developed by Mother Nature), that doesn’t mean you should abuse it. As with any pesticide, you need to know that you are targeting the right insect and you should only apply it where the problem occurs, not throughout your garden. Admittedly, Btk is less toxic than many other biological insecticides (rotenone, pyrethrum, etc.), but it can still have negative effects on harmless or beneficial insects, so it must always be applied with precaution.

My favorite “pesticides” for treating caterpillars, for example, remain hand picking (for sporadic cases like tomato hornworn) or, when the problem is recurrent, covering the plant susceptible to caterpillars (especially crucifers like cabbages and kale) with floating row cover immediately after planting, thus keeping them off entirely. Still, when you’re hit by an unexpected caterpillar attack, Btk might well be the most environmentally acceptable choice for stopping it.