Fungus gnats

rmontcalJanuary 16, 2013

I admit it - I overwater.

As I result I battle fungus gnats from time to time. Right now, all of my houseplants are succulents and cacti. I've been sitting them on the radiator to see if they like the warmth. Shouldn't that help with the gnats as well, both because of the drying effect as well as the increased soil temp? Discuss; and thanks!

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DWD2(10a, Sunset 17)

rmontcal, A professor of horticulture told me there are only 2 types of gardeners, those that under water and those that over water.
I read a lot of comments on this and other forums that "over watering" is the cause of fungus gnat problems. It is true that they require a certain level of moisture in potting media to have their eggs hatch and the larvae go through their stages of development resulting in new adults. However, that range of moisture is broad, ranging from what is suboptimal to excessive for plants. Big greenhouse operations with very expensive equipment to maintain optimal moisture levels in their growing media still get fungus gnat problems. Over watering can cause other problems, but it is, at worst, only a contributing factor in any fungus gnat problem you may have. I would avoid trying to control them by trying to "dry out" my potting soil.
There are a lot of ways that can help control fungus gnats. Bounce dryer sheets do the job for me. Others use predatory mites to good effect. Bacillus thuringiensis containing products like Mosquito Dunks can be effective. One really clever solution I saw recently was to mix carnivorous plant pots around the pots with gnat problems. I am attaching a link to a good (IMO) discussion on fungus gnats on another forum. I am also attaching links to a series of articles written by experts on controlling fungus gnats in greenhouse operations that you may find useful. I can direct you to scientific journal articles on this problem if you so desire. They are more highly technical than the links below.
Good luck with your plants!
http://www.gpnmag.com/sites/default/files/grower%20101.pdf
http://www.gpnmag.com/sites/default/files/02%20Fungus%20Gnats%202.pdf
http://www.gpnmag.com/sites/default/files/024_0710gpn1_0.pdf
http://www.gpnmag.com/sites/default/files/11_000_gpn0711_Cloyd%20JR.pdf
http://www.gpnmag.com/sites/default/files/Cloyd2.pdf
http://www.gpnmag.com/sites/default/files/010604.pdf

Here is a link that might be useful: fungus gnat discussion

    Bookmark   January 17, 2013 at 12:44PM
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dan4279

I've used bacillus thuringienus for the past two winters and both times it eliminated my fungus gnat problems within a few days. The product says that you may have to apply more than once, but once has been all that I've needed. I would recommend it based on my experience.

Dan

    Bookmark   January 17, 2013 at 8:11PM
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theficuswrangler(9/10)

DWD2 - How do you use dryer sheets to control FG's?

    Bookmark   January 19, 2013 at 9:52AM
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digdirt2(6b-7a No.Cent. AR HZ8 Sun-35)

How do you use dryer sheets to control FG's?

From what I have read it is more of a repel than control thing. Cut strips of the unused ones to lay in and around the plants and the odor supposedly repels them.

I have great luck using the dissolved Mosquito Dunks in water. Doesn't take much.

Dave

    Bookmark   January 21, 2013 at 10:41AM
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DWD2(10a, Sunset 17)

Marlie, Functionally, fungus gnats have caused problems for me when I am rooting cutting or doing starts from seeds. The larvae can really eat those tender roots. All anyone is doing is trying to drop their numbers to a low enough level to not damage plant growth. Dryer sheets have worked well for me. Depending on the pot size, I tear off a piece small enough to sit on the media surface & put it there. I am not exact about it. Often some of the sheet will drape over the edge. For stater trays, I just sit a couple of whole sheets around the tray so I can remove them when watering. I probably change the sheets every couple of weeks. When I walk into the area where my starts are or where my cuttings are rooting, I see if I can still smell the sheets. If I can not smell them, I put fresh ones out.

As I mentioned above, Bacillus thuringiensis also works to control gnats. Below is the link to a paper describing the degree to which Bt lowers the level of fungus gnats in media and the effect that both concentration of Bt used (lower is better) and frequency of use (more frequent is better) impact fungus gnat populations. This fits with the second paper linked below showing that Bt does not impact fungus gnat second and third instar larval stages which are the stages eating your roots. So, whatever the mechanism of action of Bt in controlling gnats (it too could be a "repelling" type reaction), it appears you need to use it on a routine schedule.

To be clear, I have never used Bt. My understanding is from all the reading I have done on fungus gnats. The Bounce sheets give me control I am satisfied with. As I said above, there are other methods of control that may work better in your hands.

http://www.ww.nzpps.org/journal/54/nzpp_540420.pdf
http://onlinelibrary.wiley.com/doi/10.1002/ps.1143/abstract?deniedAccessCustomisedMessage=&userIsAuthenticated=false
http://hortsci.ashspublications.org/content/45/12/1830.full.pdf+html

Good luck with your plants!

    Bookmark   January 22, 2013 at 1:33PM
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DWD2(10a, Sunset 17)

Marlie, Functionally, fungus gnats have caused problems for me when I am rooting cutting or doing starts from seeds. The larvae can really eat those tender roots. All anyone is doing is trying to drop their numbers to a low enough level to not damage plant growth. Dryer sheets have worked well for me. Depending on the pot size, I tear off a piece small enough to sit on the media surface & put it there. I am not exact about it. Often some of the sheet will drape over the edge. For stater trays, I just sit a couple of whole sheets around the tray so I can remove them when watering. I probably change the sheets every couple of weeks. When I walk into the area where my starts are or where my cuttings are rooting, I see if I can still smell the sheets. If I can not smell them, I put fresh ones out.

As I mentioned above, Bacillus thuringiensis also works to control gnats. Below is the link to a paper describing the degree to which Bt lowers the level of fungus gnats in media and the effect that both concentration of Bt used (lower is better) and frequency of use (more frequent is better) impact fungus gnat populations. This fits with the second paper linked below showing that Bt does not impact fungus gnat second and third instar larval stages which are the stages eating your roots. So, whatever the mechanism of action of Bt in controlling gnats (it too could be a "repelling" type reaction), it appears you need to use it on a routine schedule.

To be clear, I have never used Bt. My understanding is from all the reading I have done on fungus gnats. The Bounce sheets give me control I am satisfied with. As I said above, there are other methods of control that may work better in your hands.

http://www.ww.nzpps.org/journal/54/nzpp_540420.pdf
http://onlinelibrary.wiley.com/doi/10.1002/ps.1143/abstract?deniedAccessCustomisedMessage=&userIsAuthenticated=false
http://hortsci.ashspublications.org/content/45/12/1830.full.pdf+html

Good luck with your plants!

    Bookmark   January 22, 2013 at 1:35PM
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theficuswrangler(9/10)

DW - Thanks for the good answer. I've been an interior landscaper for 30 years, and my problems with FG's center not around root damage, but customer service damage - bugs flying into people's mouths when they're talking to clients just doesn't make folks happy, you know? I retired from "active duty" a couple of years ago, so now I have time to get to know many more plant people on the internet. Anyway, the pesticide that we used most recently for FG's was Gnatrol, which, if I'm remembering correctly, the active agent is similar to that in mosquito dunks. Or maybe I have that mixed up. Anyway, thanks again.

    Bookmark   January 23, 2013 at 10:02AM
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greenman28 NorCal 7b/8a

There are controls, and then there are solutions ;-)

The most effective method of eliminating fungus gnats is to eliminate the decomposing organic
matter upon which they feed - namely, the peat component found in a majority of potting mixes.
(As DW notes, the larvae will also feed on tender seedling roots).

Josh

    Bookmark   January 23, 2013 at 11:03AM
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DWD2(10a, Sunset 17)

Josh, Actually fungus gnats feed primarily on fungal hyphae, not decomposing organic matter. You are correct that media containing peat and/or composted bark support the growth of fungi very well. So, a media lacking components that support fungal growth should not attract fungus gnats. The first link in my first post above is written by 2 of the leading experts on fungus gnats. They go over this in the publication. You may well prefer media mixes without fungal growth supporting components, but a lot of those components have real benefits in various growing mixes. Since there seem to be a number of effective methods for control, you may not need to limit your media choices.

Good luck with your plants!

    Bookmark   January 23, 2013 at 11:11PM
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dan4279

I grow every one of my plants in the gritty mix and still get fungus gnats. I treat them with Bt and that takes care of it for the most part. So I do have to question the idea that changing your soil and/or watering habits is the only answer to fungus gnats. Maybe it's different from climate to climate, but for me I use a well aerated mix, water infrequently, and still have had issues with FG.

Dan

This post was edited by dan4279 on Wed, Jan 23, 13 at 23:41

    Bookmark   January 23, 2013 at 11:40PM
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greenman28 NorCal 7b/8a

Thank you for the correction, DWD2!

Josh

    Bookmark   January 24, 2013 at 10:39PM
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