I did put Bti (via ground up mosquito dunks) in my worm bin when I had a fungus gnat infestation last summer. I have no idea if it helped or not. My sense is that it's very difficult to get the Bti dispersed enough throughout the bin material - mosquito dunks are, after all, designed to dissolve super-slowly in water. I put some chunks of them into a spray bottle of water and sprayed the surface of the bin material frequently. The chunky stuff mosquito dunks are made of clogged up the spray bottle pretty often.

I later did get rid of the fungus gnats entirely after I draped a big piece of mosquito netting over the bin and tied it in place. This is the only method that I feel confident in recommending. It takes a couple of weeks but it interrupts their life cycle so that the adults can't fly out, mate, and fly back in to lay eggs. It also keeps fruit flies from entering the bin.

Thanks for your feedback. I’m finding the same out so far. I applied BTI a week ago, via Mosquito Bites saturated in a spray bottle. I also covered with landscaping fabric. I think the landscaping fabric is doing better than the BTI. I wonder if the BTI is a live bacteria culture or just dead spores? A living BTI culture in a worm bin I would think would be a permanent solution. Any ideas on that?

I tried to find out info about BTI at the time, but I didn't really feel like I knew enough to figure out a "right" way to use it in a worm bin. Apparently it was originally discovered as a soil bacteria, which theoretically could mean that it could live in a worm bin long term and maybe disperse all the way through the bin bedding - meaning it would stick around indefinitely. But I cannot think of a way for the average home hobbyist to verify that the bacteria are successfully living in the bin and that there are enough of the bacteria in the bedding to actually kill fungus gnat larvae.

The "Mosquito Bits" brand, which you used, seems maybe more likely to work in a worm bin than Mosquito Dunks, since at least the "bits" are designed to be used in the soil of potted plants. I don't see the bits in stores very much; mosquito dunks seem to be a lot easier to find. Amazon seems to have the bits.

As far as I could find out, Bti only kills mosquito larvae and fungus gnat larvae and it's very selective, so at least it's probably safe for the worms and other bin critters. But I wish I knew whether it was worth the effort.

Thanks for your thoughts. I’ve already purchased the bits and put them in action, so I guess I’ll find out one way or the other. I like to leave the worm food at the top (unburried, but covered with shredded paper) for the reason of limiting the chance of anaerobic bacteria growing, however that practice clearly opens door for flies to have a hay-day. Hopefully the BTI bacteria will take hold and dominate. If so, i’ll share my results on here.

Thanks, I now have a bug zapper. It’s the best solution I have found after trying all others that are all over the internet.

In the home apple cider vinegar works well for me. This is for fruit flies only. Put some in a dish with a drop of dish soap, cover with plastic wrap poke holes in it. FF go in and dont come out as they will drown in the liquid. For gnats I use sticky cards. For house plants you could use systemics in the soil which will kill any eggs laid , but I wouldnt use it in a worm bin.

Yeah, tried that. Bug zapper works best.

Are fungus gnats the larger flies that I see in my bins? They tend not to fly out of the bin as much as fruit flies, preferring to stay close?

Everything has its pluses and minuses. Sheets of yellow, sticky fly paper worked great against fungus gnats but also caught a tiny Hummingbird (which probably thought it was going to have a great meal of all those bugs sitting on that yellow board). Also a bug zapper worked great on fungus gnats and fruit flies. Set it near the wormbin at night. Turn it on, remove the bin cover, and stir up the bedding. the bugs fly out and get zapped. But it will also kill lots of beneficials and pollinators, like bees, ladybugs and praying mantises that are attracted to the light.