Fungus Gnats

[Gee.S]

I suspect these are not such a problem except in dry desert areas. I get them all the time, especially in my indoor seedling areas, and I see them elsewhere in my home from time to time. The problem is the larval form. Eggs are deposited in damp soil, and larvae feed on fungus and plant roots. Not much of a problem when numbers are small and plants are robust, but large numbers can lay waste to seedlings rather quickly. My preferred method of control involves a finger on the trigger of an insecticidal soap spray bottle while watering or blowing across soil. Adults fly up when disturbed, where they can be easily eliminated. If you see movement in soil, spray that as well. If they start getting out of hand, an imidacloprid drench should pretty much resolve the problem -- until next time. In my case, more fly in from outdoors, so an ongoing issue.

Edit: Skip this nonsense and see Post #7.

[Spination]

The only trouble I have ever had with these is always in conjunction with seedlings. Apparently, these insects are not attracted to soil that doesn't stay wet (so all my "regular" potted plants that are allowed to dry in between watering), and I have only ever seen them appear in seedling containers, as these are kept moist for the seeds to germinate and then the seedlings to get to size. It always mystified me, as I could start with no flies seen at all, in an enclosed room, then start a batch of seedlings, and just like that, they magically appear.
I concluded using simple logic that they had to be coming out of the soil I was buying. It seemed to me that the soil must have eggs that subsequently hatch when the right conditions were added - wetness, and I'm sure the seedling heat mats beneficial to the insects too. I started microwaving all seedling soil (made wet first) before sowing the seeds in the container, and voila! - the #s of flies seen hugely reduced, down to from zero to the rare one or two. Since I have been microwaving soil without exception on every new batch of seeds I sow, it's never been anything of a problem again such as how it was early on. Also, just to make sure that they aren't introduced externally and causing new issues after the fact, I've found those sticky yellow cards for catching white flies found at the garden center hung below the light attracts and catches the odd fly that may wander inside the greenhouse when doors are open during the day. Lastly, if I see any activity inside any container's soil whatsoever (very rare now) at any time, I break out the Sevin and mist en masse above all the containers. Several years running now without really having to deal with this problem. Nice!

[Gee.S]

Good observation regarding bagged soil, I'm sure that's spot on.

[DesertDweller]

Good observation regarding bagged soil, I'm sure that's spot on.

I will second this. I have seen the same exact thing, most predominately on the porch in some of my shadier plantings. I actually had one bag of cactus mix that, when re-opened after a heavy rain, was crawling with these things. I have never had that happen before, but after that I connected the dots and finally realized why newly potted plants on the porch (where things stay wetter, longer) were often over-run with these things in short order. Prior to that I thought it was something about the porch or an infestation there, but nope.

If the microwave fails, or you want to do a big bag in one go, I have another suggestion that has seemingly worked for me, at least in the summer. Take the bag of soil and lay it out in full sun somewhere and let it roast for a day or two. I did this on my concrete walkway around the side of the house. Got ridiculously hot, but seems to have cooked the little savages nicely.

[toditd]

And I will third it. I opened a bag of cactus mix and noticed the gnat hitchhikers. I did what DesertDweller did and dumped it out and spread onto a blue polytarp to cook it in the sun for a few days with great success. I understand that this not only drives off the gnats, but the sunlight and drying out also helps to destroy the fungus that the gnat larvae feed on.

[Spination]

Thanks for the additional info. That pretty much nails it down. I had never seen them actually active in a freshly open bag of soil, but then again the bags I used I always buy when dry. My hypothesis was the only one that made sense to me, but it was always just a strong suspicion. That you guys have actually seen them already in a newly opened bag of soil makes it conclusive as far as I'm concerned.

[Gee.S]

I have now discovered the very best cure if fungus gnats are a problem in your home -- Bacillus thuringiensis israelensis (BTI). I had used BT before, but was only recently made aware of BTI, a similar agent commercially available for mosquito control. Turns out it is also extremely effective on fungus gnat larva. This comes in a powder form, and is added to water to apply as a drench.

[DesertDweller]

I have now discovered the very best cure if fungus gnats are a problem in your home -- Bacillus thuringiensis israelensis (BTI). I had used BT before, but was only recently made aware of BTI, a similar agent commercially available for mosquito control. Turns out it is also extremely effective on fungus gnat larva. This comes in a powder form, and is added to water to apply as a drench.

Nice. Will have to give this a try.

[Gee.S]

Yeah, I picked up a suitable quantity on eBay a couple weeks ago. I still have a few scattered fliers about, but no noticeable soil activity. I'll continue to use it during normal watering until the last flier is gone, hopefully just another week or so.

[Azuleja]

Good info, thanks.

[Gee.S]

For the first time in many weeks now, no more flyers near the plants, so while I was able to manage fungus gnats by more conventional means, it was BTI that knocked them out. And a better, safer product for indoor use.

[rts265]

Ok there a a bunch of little black flies flying around my Medusa. Dumb question- bti does not damage the plant?

Is this the right stuff?

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[mcvansoest]

BTI does not damage the plant.

And yes that looks like the right product. There are a variety of forms it comes in. I have seen the powder, the little bit, and concentrated solutions you add to your regular watering can or setup.

We have used it in combination with these little UV light sticky traps, to completely eradicate them from any of our house plants.

[Stan]

What helps too is mulching the pots with anything that stays dry. Perlite or silver sands. Doesn't take much to prevent fungus gnats from laying eggs.

[DesertDweller]

What helps too is mulching the pots with anything that stays dry. Perlite or silver sands. Doesn't take much to prevent fungus gnats from laying eggs.

To each his own, but I've only had limited success with that. My house plants are very, very heavily mixed with pumice and already have a base of pumice mulched cactus mix to begin with. Did me exactly zero good with fungus gnats. I realize there are other factors, but if mulching and keeping the soil dry were enough, I'd be golden. I wasn't, more like infested at one point. I'm not a chemical fan but there was simply no other way that I could find to be rid of them.

[edds]

I would absolutely recommend the scariad fly nematodes if they are available your side of the pond. A single application last year got rid of them in my indoor growing space until I repotted some plants in fresh multi purpose compost and reintroduced their eggs.

[Epiphyte]

When life gives you fungus gnats, grow Pinguiculas...

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[Stan]

I forgot what I usually say to that problem. Set up a oscillating fan. Air movement is what those gnats hate. Try it,it works. Not only keeps the air fresher but the topsoil is properly dry.
I've learned things to the point I've forgotten what I knew.

[Aeonium2003]

When life gives you fungus gnats, grow Pinguiculas...


Pinguicula_20200710_195649.jpg

Maybe not as effective as Pinguiculas, but I do keep a few of these around my seedling trays: