Fungus Gnats

CopperheadRoadJune 5, 2012

I have been at war with them for 6 months, and have run out of ideas.

I have 4 plants in my office: a Peace Lily, a Croton, an Ivy, and a Spider Plant. They are pretty good size.

Here's what I've tried...

1) I cut water down to a minimum, not allowing water to stand in the pan at the bottom. Watering about once a week.

2) I put sand in the top inch or so of soil, and covered that with cedar mulch.

3) I soaked mosquito dunks in the water. Then I started crumbling them into the water. I did this for a few months.

4) I put yellow sticky traps out, lying them flat on top of the mulch. They catch them in bunches.

I did all of the above for 3-4 months, and it did not help.

5) I quit using the mosquito dunks and instead started watering with a solution of 1 part 3% Hydrogen Peroxide to 4 parts water. I did this for a month or so.

6) Then I decided to try watering from the bottom up (with the Hydrogen Peroxide solution) for the last 2 months, thinking if I kept the top of the soil dry it would stop them. But they are somehow still multiplying.

The gnats are just as bad now as they were 6 months ago. I replace the sticky traps every month or so, and within week or two they are covered with dozens and dozens of them.

I'm out of ideas. Any other suggestions?

My last resort is to take them all home and wash all the soil off the roots and repot completely with new soil. But I've been putting off doing it because they are in heavy pots and it would be a significant undertaking. I work on the 11th floor of a highrise building, and would have to carry them a long way. Plus, I don't want them going into shock and losing all their leaves from stripping off all the soil.

They otherwise appear healthy.

Thanks for any suggestions.

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goddess9(7b)

What kind of soil are you using?

I've personally doing a complete soil re-pot and none of them defoliated.

    Bookmark   June 5, 2012 at 7:15PM
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stonesriver(6B Tennessee)

Have you tried BTi, (Bacillus thuringiensis subspecies israelensis)?

I found it when I searched "fungus gnats."

Linda

PS: I think rhizo has used it but can correct me if I'm wrong.

    Bookmark   June 5, 2012 at 8:19PM
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rhizo_1 (North AL) zone 7

Linda, the mosquito dunks that copperhead mentions are Bt-I...just in a different formulation than the liquid. The dunks usually do the job. I can't help but wonder about the potting medium.

copperhead, are you soaking the pieces of dunks long enough so that they begin to disintegrate into the water? Gnatrol is the name brand of one of the liquid forumulations.

If the potting medium is dense, peaty, and mucky then I'm not sure what your options would be but to repot. Also, if there are any other plants in the office that could be contributing to the gnat population, you have a source of constant contamination.

    Bookmark   June 5, 2012 at 10:17PM
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stonesriver(6B Tennessee)

Thank you. :-)

Linda

    Bookmark   June 5, 2012 at 10:41PM
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RKWilcox

First confirm both the plants that are infected and that you are not trying to eliminate a fruit fly not coming from the plants. Place a slice of raw potato on top of the soil. Fungus gnat larvae in that plant will have them on the back side of the potato in a few hours.

Second use the 30% solution of Hydrogen Peroxide, mixed one cup to two gallons of water, not the 3% solution. This will kill the larvae.

Third, the sand will not work. You need to use Diatomaceous earth. Not the variety used for swimming pool filters, but the agricultural variety.

Fourth if all else fails, however the above methods will work, you need to use a beneficial nematode, and the one that works the best are Steinernema feltiae. Google where to purchase Steinernema feltiae and you will find several sources.

All of the above methods will work.

Good Luck

    Bookmark   June 6, 2012 at 12:21AM
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rhizo_1 (North AL) zone 7

RK, I'm curious....why would fruit flies be a source of infestation for the potting mix of indoor plants? Never heard of such a thing...fruit flies deposit their eggs in the flesh of vegetable matter, not the soil.

    Bookmark   June 6, 2012 at 6:22AM
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teengardener1888(NY Albany 5a)

rhizo 1,

fruit flies can reproduce in soil matter too. think about it, if he uses garden soil or compost they throw organic matter such as FRUIT and VEGETABLES in a compost pile to make the mix. one time i used miracle grow garden soil to repot my blue walking iris and bam, two weeks later both fruit flies and fungus knats in my plant and i can tell the difference between the two in adult age. i dont know if it will work but he can water his plants with a mixture if of one tablespoon of dish detergent and one quart of water and water his plants in that mixture. the soapy water will suffocate SOME but NOT ALL the larvae to reduce their numbers

    Bookmark   June 6, 2012 at 11:28AM
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rhizo_1 (North AL) zone 7

Teen, in your example the fruit flies are STILL reproducing in vegetable matter. They do not and will not lay their eggs in the typical potting medium. Most of us don't use fermenting (rotting) vegetable scraps in our potting mix.

    Bookmark   June 6, 2012 at 3:49PM
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rhizo_1 (North AL) zone 7

Teen, in your example the fruit flies are STILL reproducing in vegetable matter. They do not and will not lay their eggs in the typical potting medium. Most of us don't use fermenting (rotting) vegetable scraps in our potting mix.

    Bookmark   June 6, 2012 at 3:50PM
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birdsnblooms

Copper. You said your plants are in large pots, and you've cut back watering, from? to once a week.

Since the containers are large, do they really need weekly waterings? Weekly after you've cut back.

I'd bet lower soil is staying moist, adding water on water.
Did you ever test soil midway to bottom of pot?

Another possiblity is air. Gnats dote on moist soil and dry, stuffy air.

Since you're in an office, this would be difficult, but weather permitting, opened windows would provide air circulation. During winter months I run a ceiling and/or rotating fan a few hours each day.

Did and when was the last time you tested lower soil for moisture?
Easy to do. Insert a long stake deep within. If the stake comes out moist/muddy, lower soil is wet, and should not be watered. If it comes out dry, 'stake should be inserted in different areas,' then it's ready for a drink.

Toni

    Bookmark   June 6, 2012 at 4:16PM
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RKWilcox

Rhizo....My posting says "fruit fly not coming from the plants". I have seen many cases where someone thinks that they have fungus gnats, however further investigation has shown that those damn little bugs have been fruit fly not fungus gnats. And this has all happened in enclosed office buildings. That is why I suggested the raw potato method of finding out which plant(s) are infected.

    Bookmark   June 6, 2012 at 5:53PM
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rhizo_1 (North AL) zone 7

Gotcha, RK. Yes, lots of people have mistaken fruit flies (originating from that basket of fruit somewhere) for fungus gnats. Happens a LOT!

Fruit flies can be 'baited', too. My favorite is a little apple cider vinegar with a few drops of dish soap. It'll catch all of the fruit flies around, but the gnats won't be the least bit interested.

    Bookmark   June 6, 2012 at 10:52PM
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stonesriver(6B Tennessee)

CR:

Meant to say I love your handle. One of my favorite songs and favorite artists.

Linda

    Bookmark   June 6, 2012 at 11:35PM
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CopperheadRoad

Thanks for the replies.

I don't remember what soil I used. It is probably a cheap potting soil from Home Depot.

I don't think I am watering too much, although I may have been in the beginning when the infestation started. The Peace Lily in particular tells me when it needs water. It goes dramatically limp. I don't water it until I see it starting to droop significantly. I just tried inserting a stake to the bottom of all of the plants and it came up dry.

I am pretty sure that these are fungus gnats, but I will try the potato test to make absolutely sure.

I think I am going to order some of those Steinernema feltiae nematodes and try them. If that doesn't work, I think I will just take them home and repot them.

    Bookmark   June 7, 2012 at 12:54PM
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socks

I'm having the same problem as yours. I would recommend you do your last resort--take them home and repot them. But--DO NOT carry them down by hand. Surely in your building you have custodians who would loan you a cart. Or do what I do, use a luggage card like the one linked below. You can put a cardboard box on it and put the plant(s) in to take to your car. The carts are inexpensive, lots cheaper than medical care for a back injury. I have used them on my job and the cart is a lifesaver! I bet if you get one you would use it for other things too.

OK--off that soapbox! I went to a good nursery today, and the nurseryman recommended a lighter mix potting soil (LGN) than the Miracle Grow I used. So I'll unpot, clean pots and wash off a lot of the soil, and repot today. He also said if you have a little plant in a big pot, there is a lot of soil just sitting there wet which promotes mold and gnats.

Good luck with your problem. It's nice you have plants at work; makes it so much more pleasant, doesn't it?
(Don't carry those pots down!)

Susan

Here is a link that might be useful: luggage cart

    Bookmark   June 8, 2012 at 1:49PM
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