I'd like to clarify the directions about mixing as stated above. There are many manufacturers of commercial insecticidal soap and it's essential that the specific label directions be followed carefully for whatever product you decide to purchase. It's also important to note that dish soaps should not be substituted for the commercial product.

I don't like drenching potting soil with soap solutions because it changes the texture. It will alter the surface tension around particles, which can pose a problem in the overall porosity of the medium. But it probably does kill fungus gnat larvae.

Another topical solution is Bti....Bacillus thuringiensis israelensis, used for many years to control fungus gnat larva and other pests. It can be purchased as mosquito dunks or as Gnatrol.

The best solution is to prevent these pests in the first place by using only coarse textured potting media and by being careful about how you water. Peaty mixes can be problematic. Adding lots of perlite, pine bark fines, etc. Can help make your potting medium unsuitable for fungus gnat breeding. My pre packaged medium, which I have used for over 20 years, has produced nor fostered a single fungus gnat in all these years. Its main ingredient is pine bark.

This post was edited by rhizo_1 on Mon, Mar 31, 14 at 11:43

rhizo1--I have been using this method for the last 15 years to get rid of the gnats.

1) It doesn't matter which insecticidal soap you use. 1/2 teaspoon is enough to kill the fungus gnats. In a pinch I have also used slightly less dish soap with no harm what-so-ever

2)It does NOT make any change to the soil. It is exactly the same as before using the soap. (where you came up with that one I'll never know)

3) If you read my post right you will see that you don't have to "drench" the soil only water it until it's moist.

4) your prepackaged medium may be readily available to you but not to everybody. In my 30+ years of gardening I have never seen such a mix anywhere

Well, I already bought the yellow sticky traps before I read your post about the insecticidal soap, so...

I put out a trap and so far only a few gnats have met their demise. I'll give it another day or two for them to find it, if it doesn't work then I'll have to give the soap a try, because dang do they multiply fast! :0(

Rhizo,
Thank-you for your very reasonable suggestions and for the information re Bt.

Martha

Update: I checked again this morning - lots of gnats on the sticky trap! I think I got most of them.

What I did was shake the seedling trays to stir up the gnats, and as they were flying around I moved the trays to another room before they settled again. I then turned off all the lights on the cart except the one where the trap was hanging to lure them over there - viola!

I did find one in a seedling tray this morning who escaped death by sticky paper, but he (she?) got mashed.

Hi mxk3,

You still have to worry about the fungus gnat maggots in your growing medium. Besides being slightly annoying, the fungus gnat adults don't harm your plants. But they do mate and lay eggs that hatch into more maggots that infest your growing medium. And those maggots are eating something, which may include some of your plant roots.

ZM

As long as the larvae live in the potting mix, you'll continue to see the adults. (You know....which comes first, the chicken or the egg?) Do you know what the larvae look like? Female adults lay the eggs but there are always some males flitting around doing their thing. Aren't you glad to get rid of some of the pesky things?

The sticky cards are meant to be used as monitoring tools, rather than a control measure. It's one of those things invented to aid the professional grower or farmer in keeping track of pest populations, then adopted by backyard growers.

Should you be interested, you can probably buy the Bti where you obtained the sticky cards. :-)

Well, the gnats came back in full force. So I bought some insecticidal soap and mixed according to directions above - I'll see how that works. Thank you for the tip dowlinggram!

I was in the same situation,,I already had house plants, 3 different types of Ivy, a Yucca, and tree palm. I was given a Peace lily from a friend. 1 week later, little gnats flying around.
I removed 2 inches of the potting mix from ALL the indoor plants and topped them off with sterile sand, topped with decorative rocks. Gnats can't get thru the sand nor can the larvae. I thought I won the fight, but my containers are the "self watering" type.
gnats just flew in where you water from the bottom. Taped all openings and let the plant dry out, so to speak.
The best thing is to break the breeding cycle..if the gnats can't get in, no eggs,,,and the sand prevents new ones from getting out. I also used sticky traps around the plants and on the windows..caught around 100+. That's the adults. After about a month, changing traps etc, I'm just about gnat free. 100, 40, 10...So far so good.

I am an idiot...all this time I had 5 red solo cups growing basil on my window sill, with lil holes on the bottom for drainage,,I tapped the cups and gnats came out. I essentially gave them a back door to get in. Put them outside on the porch.
I'll follow up later.
Happy Growing.

As mentioned earlier, mosquito dunks are effective control for fungus gnats. I've only ever had the problem with them once, when I bought a bag of soil that contained them.