Fungus gnats

This is my first post as I just joined this forum today. I have a small balcony garden and most of my containers are infested with fungus gnats. I went to Home Depot yesterday and asked what I could do to get rid of them. The clerks were very kind. They suggested as you did (drying out the soil) they also suggested that I fill a gallon jug with water and add 2 tablespoons of Baking Soda. They told me that they received this formula from UBC Botanical Garden. Does this really work? I was letting my soil dry out until I used that formula last night, however the darn gnats are still there. They are really driving me "buggy" and I want to get rid of them once and for all. Your input will be very much appreciated.

 

Fungus gnats are a considerable problem when they become established. To learn more, I recommend reading the excellent synopsis entitled "Fungus Gnats, Shore Flies, Moth Flies and March Flies" on the University of California Statewide Integrated Pest Management Program website (click here).

At UBC Botanical Garden we use Drosera capensis, a southern African carnivorous plant -- Cape sundew -- to control fungus gnats in greenhouse environments. We find it to be an effective (and fascinating) management tool that is easy to grow. This species is not winter hardy out-of-doors in the Vancouver area, but it is one of the few cultivated species that does not exhibit a winter dormancy. Cape sundews are also among the easiest carnivorous plants to find commercially because the grow easily from copiously produced seed.

We grow our sundews in four-inch pots right next to crop plants on the greenhouse bench. The pots are placed in saucers that are kept filled with clean water. It is best to avoid overhead watering and keep fertilizers away from the plants. Flower stalks are frequently formed, but these should be removed before flowering, as their development reduces the vigour of the plants and the size of the leaves. It is wise to allow plants to flower once in a while, so that new seedlings can replace older plants. Fungus gnats are surprizingly highly attracted to the sundews (a new sundew in an established fungus gnat poulation will quickly generate a suicide-vortex of gnats) and the sticky leaves quickly become clogged with their pathetic little bodies.

At this time of year, Cape sundew and a number of more temperate sundew species would probably grow easily on a balcony. However, if sticky plants or tiny, dead carcasses slowly dissolving before your eyes are not attractive thoughts, try daily vacuuming immediately above the soil surface. Remember that the fungus gnat adults are not damaging in themselves, but their presence indicates a bigger, hidden problem. Continuous adult removal will eventually fix or at least reduce the problem.

Baking soda and water is used to change the surface chemistry of plant surfaces to discourage the establishment of fungal pathogens (it is an excellent control for black spot on roses, for example). I have not heard of such a treatment being effective against fungus gnats; in fact, applying such a solution to the soil would probably have an adverse effect on plant growth.

Good luck!

 

Douglas, thankyou for taking the time to explain all about fungus gnats and how to get rid of them. You seem quite knowledgeable about this pest, so I'm going to see if I can find some of these sundew plants. I'll not be using baking soda again. I really appreciate all your help!

 

Hi Douglas, I read that entire article you recommended on Pest Control Management from the University of Californa - It was interesting and I learned a great deal - thank you.

These gnats really got out of control. I went to a reputable garden centre in Abbotsford yesterday and the "qualified" nursery worker recommended that I use an insecticed called Malathion 50.

She told me to put 1 tsp. - 1 gallon of water and drench all my pots. When I got home, I read all the instructions in the little booklet that comes with it. I went ahead and did it. After I drenched each pot (12 pots) I placed a yellow sticky stick in each one. "Afterwards" I did a Google search for Malathion. Yes-it kills the gnats and the larvae but Malathion is highly "poisonous". I'm on a 3rd floor balcony (no pets or children around) thank Goodness. ! Anyway, when I woke up this morning, there was not one gnat to be seen, not even on the yellow sticky sticks.

Now, I'm suppose to repeat this procedure in 7 - 10 days. I don't like the idea of using chemicals at all and really feel discouraged this morning that it had to come to this. I should have just gotten rid of all my plants, it would have been sooooo much easier and then have a good cry. By the way, Malathon kills petunias. Mine are dead. I never had that many petunias but the ones I did have were so pretty and perky. I HATE gnats!

Now I'm sitting here with all the soil in my pots soaked in this stuff and don't know how long I should let it sit or should I start running clear water through the pots right away? I guess I'll have to call the nursery when they open. Well, at least the gnats are DEAD! I think I spent more money on gnat killing than I did on flowers this year. - Pretty discouraging for a new gardener - I HATE GNATS!

 

Douglas, what is the "bigger problem" that fungus gnats are an indication of?

Lisa

 

Fungus gnats love wet organic matter. They can be found in most artificial planting mixes, as well as in real soil with a high OM content. Fungus gnats are most commonly found inhabiting containers where the soil medium does not dry out.

Most commercial potting mixes contain wetting agents, which counteract the hydrophobicity of peat (most mixes are peat-based), allowing them to be more easily remoistened. Wetting agents also tend to exclude air, since they break the surface tension of water, and thus allow it to enter much smaller spaces than normal.

To be perfectly blunt, most people over-water their plants, either applying too much water too often, or too much water at any given time to compensate for not having watered frequently enough. Due to the shape and nature of most containers, a temporary ("perched") water table is created within the container after watering. This is a saturated zone, where oxygen is excluded. Re-applying water before this zone physically drops (dries out) keeps it in a saturated condition. Most plant roots will die if they remain in this zone for extended periods.

All of this (wetting agents, wet peat, container shape and overwatering) contributes to root death and better housing and chow for fungus gnat larvae (in a study, most fungus gnats surveyed preferred dining in plastic pots and eating dead and dying roots).

So, keep the soil aerated, water intelligently and fungus gnats will be discouraged. You may still have them around, but they won't be as successful. And I still recommend vacuuming.

 

Douglas,
Thanks soooo very much for providing all this information on fungus gnats. Now that I have gained a lot more insight about them, I'll certainly be careful to water a lot less. Would you recommend clay pots instead of plastic containers? Thanks Douglas!

 

Thanks for the help and the laugh Douglas! I keep picturing a bunch of scientists in white lab coats holding clipboards solemnly interviewing fungus gnats on their habitat and dietary preferences. :)

Lisa

 

Yes, clay pots would probably help to reduce fungus gnat pressure. Make sure they are unglazed pots.

 

Clay Pots

Douglas, again - thanks! Looks like I'm going to transplant a couple of plants when it cools down a bit. So, I'll be purchasing some clay "unglazed" pots.

 

Coincidentally, I am also familiar with fungus gnats, shore flies and Home Depot. I work for the third one, and have borne witness to the other two. Poison-wise, Carbaryl, available as Sevin, Ant Killer, Earwig Killer and a number of other products may be applied to the surface of soil and watered in. It will reliably kill the tiny white, wiggly larvae in the soil, feeding on the decaying peat moss and possibly plant roots. Please bear in mind that Carbaryl is poison. Therefore you are now a witch (witches know poisons) and will grow a wart upon the end of your nose. Carbaryl persists for a time in soil, killing, until it is broken down by soil micro-organisms. Its somewhat hard on fish, less so in mammals. Soil microorganisms tend not to live in the sterile media of houseplant roots. Sundews would be the most delightful answer to remove your fly concern. and can be practical commercially, but I've killed a few. Yellow sticky traps near the soil surface will catch adults, if that is what is annoying you. Personally, I have fruit flies breeding in the kitchen. Many might think their houseplants are to blame. In my case, it's the empties.
Drosophila melanogaster: I had them as pets, as a kid.

 

Woooohoooo! They're all gone! But the wart on my nose has really grown. Yep, when I watched those little buggers die, somehow I felt an overwhelming "joy" - Must be the witch in me! LOL

 

I have found that ground cinnamon dusted on the surface of the "soil" helps to greatly reduce the fungus gnat population.

 

Would it be possible for me to get some seeds of this sundew from someone at UBC? The Shop in the Garden didn't have any.

The plant that I had (which came from a greenhouse at UBC) recently took a turn for the worse. I'd like to get some replacements as it did a great job of capturing fungus gnats. It also produced some seeds but none have germinated; it has been almost 3 months since they were started.

 

Gnats

I had a terrible time with gnats. What I did was buy the little yellow sticky paper that you skewer on a peg and put in the dirt of each plant. As they reproduce in the soil.
You could also simply repot your plants and use the sticky paper as a preventative.
They are sold at all the nurseries and they really do work. Kathy

 

In my greenhouses, I`m successfully controlling fungus gnats (Sciarid flies) by using nematodes (Steinernema feltiae) and Hypoaspis miles which are to some extend also effective vs thrips

 

I had a huge (an understatement) fungus gnat problem last year, and what I found worked best was placing small trays of lemon juice in each plant. This lured in the adult gnats and as soon as the new gnats would arrive they would go straight to the juice and DROWNED! You wouldn't believe how many I caught.

I'm suprised no one has mentioned using lemon juice yet!

 

I use a "Vapona Pest Strip". Hang it up for a while until the problem is gone, then wrap it up and put it away. Does the job where all the labour intensive powders didn't. My stuff is all indoors though.

Also: I wonder sometimes what I'm doing to my own health.

What would a Cape Sundew eat when all the fungus gnats are gone?

 

I dont think the Vapona strips are available anymore around here. the chemical, Dichlorvos if I remember correct. its not in these parts anymore.

 

I don't think you would get 100% eradication with sundews - just control.

 

Guess I better look that up. I bought a pest strip as recently as last year, and I think it was at home hardware. Or crappytire.

Um...jimmyq...do you know if Dichlorvos is dangerous to old people and Bull Terriers? I don't leave it unwrapped for more than a week or two, and sometimes leave it in the germinator for a few days.

Carl

 

Arrr! I guess I could just leave the door open and feed 'em mosquitos...

 

A spray of BTK (bacteria commonly used to control caterpillars and mosquitoes) seems to get rid of thrips in one shot.

 

I love how this post has stayed alive since July 2003!!!

Hopefully those gnats are gone by now:)

 

I recently saw on the internet that a BTK solution is available in the USA specifically formulated to control fungas gnats in houseplants. Unfortunately it is not available in Canada. If I wanted to use the BTK ,sold here for mosquito larvae, for my houseplants does anyone know what dilution I would use. Or does it matter- can you add too much BTK?