Top 3 Methods for Controlling Fungus Gnats
No one likes fungus gnats. They probably don’t even like themselves (ouch!). I’m a peaceful being but when it comes to plant pests, I go to war. Today I want to show you the top 3 methods I have found for achieving and maintaining a gnat-free home. I’m sure most of you have heard of one of these, but the other two are lesser-known (and really great!). We’ll also talk about proper watering - SPOILER ALERT: it will not control fungus gnats!
(In each section below, you will also find links to the YouTube videos I have posted for each of the 3 methods.)
Control methods covered in this post:
In addition to these three methods I would also consider the use of yellow sticky traps to be mandatory. Adult gnats are capable of laying hundreds of eggs so every single adult that gets killed is a major victory. I usually just lay mine right on the soil surface/pot rim and it works very well. No need to use stakes or ties to try and prop them up. In my experience, it seems that gnats are attracted to other gnats; once the traps start filling up slightly they tend to begin filling up much more quickly. If I see gnats flying around I often pick up a yellow sticky trap and try to smack them with it. I’ve even gotten the occasional double but the triple smack has thus far eluded me. Clapping is also effective.
Let’s briefly discuss fungus gnats and why we want to get rid of them. Fungus gnat larvae live in the soil and feed on organic matter and plant roots. It takes TONS of fungus gnats to actually cause any real damage but obviously we want our plants to be as healthy as possible. Plus, the adult gnats are attracted to carbon dioxide which is why they often try to fly up your nose (annoying!). Adult gnats don’t harm plants in any way except that they are constantly laying more eggs in the soil. I’ve never personally had any plants actually decline due to fungus gnats but it is possible. Certainly they could send a struggling plant over the edge.
The biggest key to dealing with any plant pests is swift and effective action.
As soon as you see one of these nasties, let the war begin!
*Note: I am not suggesting or discussing the usage of chemical pesticides in this post; I don’t use any. All of the methods I am sharing are naturally-occurring and safe for your plants. I would encourage you to view chemical pesticides as an absolute last-resort.
Bacillus Thuringiensis Israelensis (BTI)
BTI is a microbial insecticide that kills fungus gnat larvae in the soil. It can be highly effective but it does not kill the adult gnats. This means that repeat treatments are necessary since the adults will still be flying around and laying eggs. It generally takes a few weeks and 3-4 treatments to break the life cycle.
BTI is sold in a product called “Mosquito Bits”. Designed to be used in standing water to kill mosquito larvae, it has proven to be effective at killing fungus gnat larvae as well. When the product comes into contact with water, the BTI will be released.
My Favorite Method: add several tablespoons of mosquito bits to a jar/container and fill with water. Let it sit for 24-48 hours to ensure that all BTI is released into the water. Pour this “concentrate” into your watering can and add a small amount to each of your plants.
*This is my preferred method because it allows me to easily treat my entire collection in a single day. By using a concentrated form of BTI, I can safely pour a small amount on each plant without fear of overwatering.
Frequency: repeat treatment 1x per week until desired control level is reached (usually 2-4 treatments). I always do this 3 weeks in a row just to be sure. If you have not noticed a difference by week 2 or 3, go ahead and make your next batch double-strength!
Alternate methods: sprinkle mosquito bits on the soil surface or mix into soil for new plantings. This can be effective but if you are watering your plants on different days (as you probably should!), there will always be some pots that are much more hospitable than others for the fungus gnat larvae. This allows them to continue reproducing indefinitely, in my experience.
Hypoaspis Miles (Stratiolaelaps Scimitus)
Hypoaspis Miles is a beneficial predator. These little warriors will feed on all types of soil-borne pests and can survive on decaying organic matter once the nasties are gone. I really enjoy releasing these because they can survive and reproduce for a long period of time after a single treatment. While BTI exclusively affects fungus gnat larvae, hypoaspis miles will eat any pests that get in their way.
They arrive in a container with mostly vermiculite and some food and need to be released within 48 hours of arrival for best results. You will need to pay for overnight shipping with any order of live insects so I’d recommend finding a dealer near you to save on shipping costs. I have always ordered from Nature’s Good Guys here in California and my orders have both arrived safely and thriving.
*Note: these will be shipped in all life stages. This means that you will see some adults wandering around but there will also be eggs and pupae in there as well. If you don’t see much movement at first, don’t worry; things should start picking up after a week or so.
Method of Release: The key here is to get some in all of your pots. I emptied the container into a bowl and scooped a teaspoon onto each of my plants. As it turned out, I had enough to do this three times over and so I did. I like this method because it ensured that I had enough for my entire collection.
Frequency: Well, I suppose you could release them multiple times but I have gotten good results by releasing 1x per year for the past two years. In a greenhouse setting I could see more being beneficial (seasonally?) but I haven’t found this to be necessary. Even 6+ months later I still see them going to work in most of my pots. Just the sight of them now makes me feel safe!
Since these will be shipped in a container that is suitable for treating 50-100+ plants (depending on the option you select), I would not recommend this method for anyone with fewer than ~35 plants. Biological control methods are really much better suited for people with larger collections.
Beauveria Bassiana
Beauveria bassiana is a mycoinsecticide. These are fungal spores that grow into and destroy plant pests. It will take a couple weeks for it to really grow and spread throughout your soil but it becomes increasingly effective as it does so. This is actually becoming my preferred method of the 3 on this list for this very reason. Once the mycelial network has populated the entire pot, no pests are safe!
The product I have used is a dark black powder. It may or may not stain your furniture/carpet so please be very careful with this stuff. It definitely DOES stain the surface of the soil for a while so, if you are a top-dresser, definitely scoop those rocks to the side and apply it carefully!
Like all mycelium (as far as I know) this forms a white, fuzzy, powdery looking network within the soil. I have found this to be present in my pots even a full year after applying it! That’s really what makes this stuff so great; it lives and grows and continues to put in work!
You can also mix this up (in a MUCH more diluted formula) and spray it onto the foliage to control other pests. I have never tried this and will never try this indoors due to the staining potential of the product. I have read that it can be effective but it must come into contact with the pests; it will not live and develop on foliage as it does in soil.
My Preferred Method: as with the BTI, I prefer to make a concentrate and treat all of my plants in a single day. I simply add 2 tablespoons to my 16 oz squirt bottle and SWIRL GENTLY! You don’t want to spill this stuff while mixing. Squirt a small amount on the soil in each of your pots and you’re done. (I generally try to apply this in a circular fashion, halfway between the base of the plant and the pot rim).
Alternate Method: sprinkle 1/4 tsp into the soil for a new plant or add 1/2 tsp to the soil surface of an established plant. The spores will be released upon watering.
Frequency: theoretically, one application is enough, but remember: we’re at war! I do this 1x per week for 3 weeks to make sure there is enough to really take hold in all of the soil.
Compatibility: this fungus does not discriminate between good bugs and bad bugs. It could potentially even harm our beloved friend, Hypoaspis Miles. However, I have found research that suggests these two to be MORE effective together, with beauveria bassiana causing little to no harm to hypoaspis miles. (*Note: this is said to be safe for bees, but do not apply directly to flowers; keep it in the soil.)
Quick Comparison
BTI
1. BTI is cheap, widely available, and has a long shelf life (years?)
2. It is very effective against fungus gnat larvae but it will not help control any other pests (except mosquitoes, which are not living in your soil).
3. The bacteria will die off fairly quickly - they do NOT persist in the soil.
4. Can be easily applied to your entire collection in a single day.
5. Repeat treatments ARE necessary.
Hypoaspis Miles
1. Effective at controlling all types of pests living in the soil: fungus gnats, root aphids, soil thrips, etc.
2. Can travel between pots, going to where the “food” is.
3. Will reproduce and persist in the soil by surviving on decaying organic matter.
4. Must be shipped overnight and released soon thereafter. You cannot save any for next time.
5. Repeated releases may or may not be necessary.
Beauveria Bassiana
1. Continues to grow and become more effective, gaining traction in the soil over time.
2. Product has a decent shelf life of ~1 year.
3. Easy to apply to an entire collection in a single day.
4. Repeated applications are not strictly necessary, but definitely recommended.
Wait a minute, don’t proper watering habits get rid of fungus gnats??
Okay, so this is a common gardening myth that I hear far too often. The short answer is — no. Watering correctly will not rid you of your fungus gnat problem. Fungus gnats prefer to live and lay their eggs in moist soil, so many people claim that by simply allowing the top few inches of soil to dry out, all the gnats and larvae will magically disappear! This is both ridiculous and insulting to the professional growers who spend millions each year dealing with these pests.
Fungus gnat larvae are living, mobile creatures. If the top layer of soil becomes inhospitable, they will migrate down further inside the pot. The adult gnats will prefer to lay their eggs in a wetter mix but they aren’t just going to give up because the soil surface is dry. I wish it could be as easy as watering correctly but it just isn’t enough to solve the problem.
So what is my plan moving forward?
Well, I actually used all three of these control methods this year but I am planning to stick with beauveria bassiana and BTI as my default strategy from now on. The fact that I can keep these on a shelf somewhere and apply them whenever needed is just a huge win. Being able to treat my entire collection in a single day is such a blessing. I especially love that beauveria gets stronger over time. Whenever I am able to build a greenhouse I will likely release all types of beneficial insects, but for now I’ll be relying solely on these two to control fungus gnats. If I change my plan in the future I will definitely let you know but that’s where I’m at currently.
I also just want to add that this was a mildly painful realization for me. I LOVE hypoaspis miles. They have put in so much good work for me these past two years and I literally get feelings of health, strength, and safety when I see them wandering around my plants. Unfortunately, they are losing out simply due to convenience and availability, but I will give them a very good life in my greenhouse one day :)
I strongly encourage you to give these methods a try because it’s such a great feeling when you realize you haven’t seen a fungus gnat in weeks!
Best of luck to you with your plants and your life!
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