Diatomaceous Earth and Fungus Gnats

It’s always been my experience/ understanding DE on the soil doesn’t work very well against fungus gnats. I was doing a little reading tonight, and I came across these two articles.

Curious everyone’s input on this. :v:

From DE-insecticide.pdf

The efficacy of DE primarily depends on the physical properties of its dust, and not on its chemical composition (Korunic, 1997). The dust is physically stable and will affect insects as long as it is dry and in sufficient concentration to ensure that insects will come in contact with enough diatom particles. Everything that reduces the ability of particles to absorb wax from insect cuticles directly reduces the efficacy of DE. An increase in moisture content of treated grain or air relative humidity (r.h.) will considerably reduce the efficacy of DE, especially if moisture content is more than 14% or relative humidity exceeds 70% (Korunic, 1994; Fields and Korunic 2000). It means that products with high moisture content should not be treated with DE or, having no other solu- tion, higher dosages should be applied. Increased temperature increases the efficacy of DE, an exception being spe- cies of the genus Tribolium (T. castaneum and Tribolium confusum Jacquelin du Val).

Effect of Diatomaceous Earth and Trichoderma harzianum T-22…

The dry and moist DE and sand treatments had no affect on fungus gnats in terms of inhibiting adult emergence or preventing egg laying by females. Adsorption of moisture by DE particles reduces the abrasive properties and thus effectiveness against arthropod pests that reside in the growing medium (Maceljski and Korunic 1971, Weinzierl 1998). In addition, during the course of the Þrst two experiments we observed that the DE dry treatments expanded, providing Þssures (or openings) that allowed the fungus gnats to pupate and females to lay eggs.

DE-insecticide.pdf (552.9 KB)

Effect of Diatomaceous Earth and Trichoderma harzianum T-22 (Rifai Strain KRL-AG2) on the Fungus Gnat Bradysia sp. nr. coprophila (Diptera - Sciaridae).pdf (89.2 KB)

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DE is hard to work with. Dont breathe it. Ineffective as soon as it gets wet.

Get Gnatrol n eradicate em first time.

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Had to work with DE and was not a fan. Can get real messy and airborne real quick.

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I am experimenting with combining spinosad powder and DE 50/50 in a plastic ketchup bottle. I turn off the fan, wait for the air to quiet then shake and squeeze a puff at the base of each stem. It sends a plume of white cloud up under the plant. Seems to be keeping the population low if not entirely exterminating. Wont want to go too far into flower with this.

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This and the mosquito dunks have both worked extremely well.

Just a few drops per gallon.

Sticky traps for the flying adults.

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I still find finely ground malted barley and neem top dress not watered in for a few days does the job along with the stickies. The added benifit of the barley and neem is the nutrients, enzymes etc… I do this in my no till and seems to work fine. YMMV

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If you’re growing in pots, put little saucers under them and bottom feed…let the very top of your medium dry out…dust with DE…bottom feed for a little longer.
Then when you want to start top feeding again maybe water, then add a thin layer of dry sand or hydroton and dust that.

Not sure if any of that would work…but that’s what I would do.
Also heard of people just capping their soil with sand…

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All I’ve ever used when I used to get fungus gnats what neem oil. I’d water the soil with a sprayer every 2-3 days, after a week they’d be gone. Once the soil gets its biology up to par you dont see them.

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I usually pour my fresh soil into a large tote box and cover it with a thin layer of DE for a week or two, before I use it. Haven’t had gnats in years.

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I agree completely! The boss is hell bent on using DE cause ‘it worked for him before’ - which I’m really not buying. We currently use sticky banner, h. miles and nematodes - which are actually working quite well. I have a gallon of microbe lift, but it doesn’t have any application rates for this type of situation. I already suggested gnatrol. :v:


I would love to just top dress with neem/karanja, but the boss doesn’t want to use neem. :thumbsdown:


We are using 20’ x 4’ x 2’ fabric beds in a greenhouse, so I don’t think the bottom watering will work. Blumats don’t help either imo lol.

Thank you everyone for your responses! :v:

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I have seemed to had good luck with a thick layer of sand to help smoother them however with my worm bins if I notice fungus gnats I apply fresh worm castings which have beneficial mites in them gnats get the fuck out of town and don’t hang around . ALSO a spray of JWA would work also but have never had to use it on them just those damn aphids on the outdoor grow.

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I completely agree. If I topdress with fresh EWC every month, I do not have an issue with fungus gnats.

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Hypoapsis miles n nematodes will work to control but will never eradicate. Sounds like u got notill setup so them natural predators should stick around forever if u treat em nice.

Microbe lift use at aquarium rates. What it sayz put in yer aquarium put in yer water rez when you water.

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How bout Malted barley finely ground down? Thats works just as well. I just add the neem on top of the barley as an extra

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I’m familiar with using malted barley for enzymes, pgr’s, pgh’s, etc - but not as a pesticide. Can you explain how that would work? Just curious.

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It dont. He misspoke. It makes yer FG worse cuz it creates a fungal bloom on top yer soil.

Use yer microbe lyft @ treatment recommended fer aquariums n overcome em.

I didn’t mispoke. It dosen’t make it worse at all.

You can go take a look through the no till revisited thread over a GS forum both coots and MO recomened using it. I use it it my garden with great success when and if I have them, but don’t take my word for it take the word of a well respected long time member here I recall that @ReikoX has reomended the same treament too.

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Sorry but yer gonna have to reply w a reference link to the xact page outta 863 pages cuz I can find no mention of malted barley used as pest control by itself errr ne action mechanism proposed for malted barley in that GrassCity thread. Perhaps u mean barley straw?

Barley err ne starchy amendment used in growing cannabis (regardless of its hormonal err nutritional content) dont have no properties of pest control. Only 1 I can think of is neem n karanja seed meal but it aint real starchy n it has leftover neem oil in it.

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Here’ the post you will have to do your own digging around those few pages yourself though for the explination from coots it’s there somehwere and like you I aint diggin through nearly 850+ pages I’ve done it already in the past :thumbsup:

[No-Till Gardening: Revisited](No-Till Gardening: Revisited)

Here’s the next post a success story MBP works I had a bit of a break out about 10 days ago and they are all gone. not even anything on the new traps

[No-Till Gardening: Revisited](No-Till Gardening: Revisited)

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Thx fer the links. So Coot said do it. But no reasoning err action mechanism n nobody asked him bout it. Why aint nobody questionin this?

I dont see no citations or ne other literature bout it being of ne benefit against fungus gnats n it aint in no commercial pest control so its hard 2 believe when topdressin w it becomes fungal in good soil. It grows fungal web on top of it like bokashi if yer soils good. Seems like itd encourage not discourage.

But I do thank u fer pointing out Coots anecdote. :thumbsup:

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