rredbbeard(SE CT USA/zone 6)April 22, 2013

Is there a mild insecticide that can be used to kill fungus gnats/larvae, especially in soil? I'm mostly interested in treating very small seedlings, so drying out isn't a great option.



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irina_co(z5 CO)

Bake the soil before putting seeds down.

Anything will suppress them - Safer soap in a water will help.
The larvae lives in a first inch of soil - you need to reach them. So - some kind of drench. Fly paper catches adults.

Without strong pesticide - you cannot really eradicate them - but you can keep the numbers way down.


    Bookmark   April 22, 2013 at 11:50AM
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rredbbeard(SE CT USA/zone 6)

Thanks--that's what I figured. I put several squirts of safer insecticidal soap in a quart of water earlier, and have been watering with that. So far so good, at least none of the plants have suffered, and the soap has a little pyrethrin in it.

I wish I still had my sundews!


    Bookmark   April 22, 2013 at 1:32PM
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You can put one mosquito dunk (available in all garden centers) in your watering container or bucket you get water from. Let the water sit for a couple hours before using (I leave it overnight). Water as usual... the ingredient in the mosquito dunk somehow affects the respiratory tract of the gnats and larvae and kills them. If used regularly, you will get rid of them altogether.

    Bookmark   May 1, 2013 at 4:35PM
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rredbbeard(SE CT USA/zone 6)

Excellent! I water gallon by gallon, wondering if I can make one concentrated gallon and then dose other gallons as I use them?



    Bookmark   May 1, 2013 at 5:25PM
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Probably would work.... I know some people that actually crumble up the mosquito dunk, and just work the crumble into the top soil. I learned about it with the watering method, and have always done it that way. I just leave a whole dunk in my 2-gallon pitcher all the time... until it breaks down.


    Bookmark   May 2, 2013 at 1:02PM
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rredbbeard(SE CT USA/zone 6)

I ran to HD (not really!) and went looking for dunks. There was a guy with the same gnat problem and we split a package of dunks. Surprised that 6 cost $10.

Such a deal!


    Bookmark   May 2, 2013 at 2:32PM
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I use one of 3 Biological control : Releases of natural enemies should be done when populations are low, at start of crop.
My first choose is Bacteria: Gnatrol

1# Nematodes Steinernema feltiae
Is the most effective nematode species for fungus gnat larvae. Make first application at planting, or soon after, then 2-3 weekly applications. Irrigate the day before application. Apply as a drench.
Nematodes are not usually good at reducing a serious infestation, one reason why they should be applied near the start of the crop.

2# Bacteria: Gnatrol is applied as soil drench. The active ingredient is a bacterium that must be ingested by fungus gnat larvae. The bacterial toxin that kills the fungus gnat larva is only effective for 48 hours after application; so repeat treatments with higher label rates at 3 to 5 day intervals for heavy infestations.

3# Predators: Releases of the Predaceous mite, Hypoaspis miles, may give excellent control. Release soon after planting. A single release has provided 6-8 weeks of control in some cases. The predators are capable of establishing and spreading throughout a greenhouse if the environment is favorable. They live in the top layer of soil and feed on any small arthropods they encounter (including thrips).

    Bookmark   May 11, 2013 at 12:55AM
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