Mosquito Dunks For Gnats and Fruit Flies – Revisited

Back in August of 2009 I decided to test out “Mosquito Dunks” as a potential way to get rid of fruit flies and fungus gnats.

My (somewhat vague) memory of the experiment(s) was that there wasn’t an obvious reduction in abundance of gnats and/or fruit flies as a result. BUT, I have recently come across at least a couple of mentions of success with this method from others, so I’m eager to test it out again (and compare to my parasitic nematode experimentation).

Here’s what I really like about Mosquito Dunks (in comparison to nematodes):

  • Dunks are quite a bit less expensive
  • They have a shelf life of who-knows-how-long (MUCH longer than nematodes)
  • It is more of a direct approach (with nematodes you rely on them staying alive, finding/invading the host, and infecting the host with the bacterium)
  • A little goes a long way – pretty sure a single dunk can keep a rain barrel full of water free of mosquitoes for quite some time.

Dunks release the bacterium, Bacillus thuringiensis israelensis (no wonder they call it “Bti”?? lol) gradually over time. Bti produces a toxin that kills various dipteran (“true fly”) larvae, while not causing any harm to various other life forms, including worms, humans, and pets!

My innner “biology geek” is still a little uncertain about my chances of success with fruit flies, to be honest. Gnats and mosquitos are in the same broad grouping (sub order) within the order Diptera – called Nematocera – while fruit flies (along with house flies, soldier flies etc) are in the sub order Brachycera. I’m still not 100% sure that Bti targets brachyceran larvae.

Only one way to find out, I guess! šŸ˜‰
(And like I said, others have reported success so I’m still somewhat optimistic)

My first test method is pretty basic. I simply broke off a small chunk-o-dunk (I had way too much fun writing that! lol) and dropped it in a small container of water.

Now I’m letting it sit for 2-3 days before I add the liquid to a test system containing loads of gnat and/or fruit fly larvae. I’ll then fill the container up with more water and repeat the process a few days later. Naturally, it will be important to balance all this liquid with some dry bedding (in my ziploc test systems) as well.

I may also just try crumbling up a dunk and sprinkling it into one or more systems. The obvious advantage of that approach is that I won’t need to keep adding more liquid to the systems – but the potential downside is that I may use up my remaining dunks more quickly.

As always, I will keep you posted on my progress, so do stay tuned!

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    • Paul from Winnipeg
    • February 27, 2017

    Watching with interest Bentley. I’ve tried this before with mixed results, but I didn’t wait as long as you before using the water. Maybe that will make a difference.

    Hope I didn’t send 2 comments by accident. Haven’t seen my last one for a few days.

    • Bentley
    • February 27, 2017

    Hey Paul – good to hear from you. Apologies for the issue with the last comment. I didn’t notice it come through and for some reason it was held for moderation.

    When you say “mixed results” does that mean you did see SOME indication of it working?

    • Bobbi
    • July 4, 2017

    I’m curious about the results of this experiment. šŸ™‚

    • Gary Scott
    • February 4, 2018

    I grow using Aquaponics. I use Red Worms in the grow beds to eliminate fish solid waste which works great BUT the fact that this is a wet enviroment Fungus gnats are an issue. My question is will Bti (Bacillus thuringiensis israelensis) kill my worms or the benificial bacteria from the fish waste?

    • Bentley
    • February 8, 2018

    Hey Gary
    I know of NO evidence to suggest that Bti will harm worms. I have played around with the dunks (adding water the dunks have soaked in or even crumbling the dunks and adding them directly, with no issues). I doubt it will harm beneficial bacteria (who are likely better adapted for that environment) either. Just my 2 cents worth!

    • Mel
    • October 20, 2021

    Any report back on how things went?

    • Bentley
    • October 22, 2021

    Hi Mel – it’s been quite some time since I tested this, but I do recall that overall this approach didn’t seem to be very effective. Others have reported success with dunks though, so I may test it out again.

    • Mel
    • October 22, 2021

    Thank you! Iā€™m having better luck with Mosquito Bits, same concept but different application. The bits are easy to distribute in the bin and have worked in the past, I may just incorporate them into the mix periodically.

    • Bentley
    • October 22, 2021

    Very interesting, Mel – thanks for sharing that. I’ll have to try those!

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