Know your enemy (Fungus Gnats)

Well we’ve covered now three big threats to anyone’s Cannabis grow, but how about something that’s even MORE ubiquitous, but thankfully falls mostly into the realm of just annoying as fuck.

Fungus Gnats. The common name encompasses many species of the Dipteran order, organized into the ‘superfamily’ Sciaroidea and all being basically the fuckin same. These are the little gnat bastards that buzz around the surface of your soil, and the lower parts of your plants, and the standing water in your flood table, and in your dreams when you go to sleep at night. Almost always present, oftentimes little more than an nuisance, but in large enough numbers they have an uncanny ability to get right under your skin (and sometimes right up your nose); not to mention potentially stunting the growth of your plants if left unchecked.


Fungus gnats have a pretty simple life cycle. Adults lay their eggs in the soil of your plants, and yes even in your rockwool or other hydroponic media. After hatching, larva squirm around the root zone of your plants being dicks, before pupating and emerging from the soil as an adult to harass you while you’re pruning. Its very difficult to completely avoid their presence, especially if youre in a greenhouse and even more especially if you’re buying in soil from the hydro store or some other distributor. Potting soil is notorious for having fungus gnat larvae and eggs in it, and if you use it you are almost certainly going to come across them. Cant really be helped, but it can be managed.

This little shit bag is fungus gnat larvae. You’ll be able to find them if you dig around in your root zone long enough, and sometimes you can even find them on your clones if you keep the cuttings in the same room. The little black part is the head, and the rest of it is just a tube for shit to pass through. Worthless creature.
However, despite being worthless, they will also demonstrate negative worth by feeding on your roots. If enough of them are treating your poor plants like a buffet you could end up with chlorotic foliage, subpar root structures, and overall weakened plants that will present with chlorotic leaf margins, tip burn, twisting foliage, any number of symptoms that might suggest other problems but really stem from damage to the roots.



Luckily, they can be managed fairly easily without resorting to expensive chemicals. First of all, you need to address your cultural practices and sanitation practices. Watch how much you water, the quickest way for fungus gnats to get out of hand is if your plants are chronically overwatered. Allow that root zone to dry down properly, and they wont have such an optimal environment to breed and spread. Secondly, don’t let leaf litter accumulate in your grow area and especially not in the plant pots themselves. Dead leaves on the surface of the soil is bad for a lot of reasons, but they’ll become a haven for fungus gnats as well. Leaf litter in the general area of the grow is also bad and provide a constantly moist and shady area for them to live in. If you’re pruning, defoliating, or whatever just make sure to bag and remove. Cleanliness is next to godliness as they say and that should keep the devils away.

If your infestation is bad or you’re just looking for for preventative measures to keep the populations very low you can try a few different beneficial creatures:

Rove Beetles (Dalotia Coriaria) – My personal favorite and an excellent predator of all soil pests. Fungus gnat larvae is like a little treat to them they love it, and they are highly mobile. There’s a relatively simple method to breed these in your grow, DM for details.

Hypoaspis Miles – A beneficial soil mite that always comes in handy. Quick to reproduce and develop a strong population in the presence of the gnat larvae. Very good stuff.

Beneficial Nematodes – Steinerma Feltiella is the go-to nematode for treating fungus gnat infestations. Combine with either Dalotia Coriaria or Hypoaspis Miles for a double-pronged treatment for greater effect. Nematodes are compatible with both (That of course meaning <25% predation on them).

I’ve been enjoying the discussions in the comments from these, so I’d like to keep doing them as the mood hits. Does anyone have any requests, even for other IPM related things?


Slf100 is micrboe I think that grows crystals in soft bodie pest. It kills them at higher doses and give utons microbes

1 Like

I tried looking it up but SLF-100 seems to be an enzymatic reservoir cleaner, maybe I didn’t look hard enough though!

1 Like

It’s secret part about the manufactor and nectar the God’s figured out. Will grow crystals in there bodies

1 Like

Got this off rollit up

looks like you have some dead plant matter on the surface of the soil in the 2’nd pic, that will attract them. Do you use SLF-100? a good TBL per gallon of that will help, it contains baccillus thurengensis, among other enzymes that kill fungus gnat larvae, it does wonders for your roots, it will help repair any damage the larvae may have caused. imo your plants look fine btw

I learned this years ago…it’s also how I flush my plants before harvest

1 Like

Another issue with fungus gnats is that they get stuck on sticky buds like it is fly paper.

A yellow sticky trap at the base of plants can help to control them, and makes it easy to see how bad the issue is.


Great callout! Can’t believe I forgot to mention sticky traps lol


Gnatrol. One and done (assuming you also implement cultural practice changes).


I have found these little bastard don’t like places where there is air movement from fans. I have small fans in all of my grow bays that create a lot of air movement at the grow pot level. I have always used wall fans that blow across the canopy but the addition of the smaller floor fans keeping the air around the floor moving seem to really help with the fungus gnats.

1 Like

Soaking mosquito dunks in your water before applying, or if in hydro, placing half a dunk in a sock in the reservoir will take care of them. There are lots of bt products that work. The dunks are just cheap.

The other tip for soil growers is to place perlite, vermiculite, or diatomaceous earth on the top layer of your soil, then bottom water the container and try to keep the top of the media dry.


Hot tip with the DE, I always sleep on it but its effective against alot of things as long as it remains dry like you said.

Another vote for Bti + DE. Rip em up with the DE and infect them with the Bti, hard to beat.


Mix Mosquito bits in soil blend.

Mosquito dunks in reservoirs all the time.

1 Like

I have struggled with fighting fungus gnats for a while not that they were causing major issues just annoying. Brewing a weekly compost tea with beneficial nematodes helps . Also i think
Slf100 is amazing . I just had some peanut butter breath that did not respond so well to the water schedule of the rest of my room . Hit them with a dose of slf and they perked right back up . It does help with fungus gnats too . Flower that is flushed with slf has a noticeably whiter ash and is smoother imo . It is expensive but i think it is worth it .

I swear the larvae comes in our soil. We have battled them for years with our ornamental production. Most soil manufacturers store there peat outside and if you have bark in your mix, Lord knows what other bugs are hanging out. I’m trying Bio 365 soil next week, they boast about no shore fly larvae…

Microble life is the best i have found. The dosage rates are minimal since its made for ponds.

1 Like

Microbe life has some really good products.

1 Like

Gnatrol and yellow sticky cards


Is gnatrol the cedar extract one? Haha


Mosquito dunks work too