fungus gnats - how to control them

Many people at some stage develop a fungus gnat
problem.
They can be tricky to get rid of.

Just wondering if anyone has any advice on controlling them?

I used a neem spray called 'neemgold' for a while. it seemed to keep the numbers down but didn't knock them down all together.

I also tried yates 'confidor' every three days this also seemed to reduce numbers but they soon were back - I think this might work with many applications.

also keeping the potting mix dry will help - they only have a life cycle of only 8 days ( what a life!) so keeping the potting mix very dry should break the cycle... in theory.

I have a small problem to, in spring I am going to introduce a predator insect called Hypoaspis Miles that feeds on the fungus gnat larvae...

Trig mentioned a product called gnatrol, would like to hear info on that to...

http://www.biologicalservices.com.au/hypoaspis.htm

i am looking for an effective method of control as well, and am yet to find one. luckily i haven't had them that bad. i did use confidor on the adults and it kills them at least, don't know about the grubs though. yellow sticky traps are supposed to be good for the adults.

I have been told to top dress the pots and also having grit in the bottom of the pots as well. Apparenly they can't get through it to lay the eggs.............haven't tried it yet though.

Did you water with the confidor or spray with it? I water with rogor when I get an infestation and have one now as well watered last weekend and some still around so hoping they will be gone soon.

Also use an aerosol spray to kill the adults.

i sprayed with it, doesn't seem to harm the seedlings at all. in fact it's systemic so it probably kills the grubs as they feed on them. i was actually quite surprised it kills the adults instantly on contact, being a soft systemic chemical.

From my exprience there are two ways to control fungus gnats. I personally had a bad........bad gnat problem last year and as of today have not seen one in several months so I guess I can help out here.

First lets look at the gnats themselves. Fungus Gnats are a type of Sciarid fly, from one to at the very most seven millimeters long. They have a slender, darkly colored body and dark wings. However, the females of several species are wingless. Their long legs and antennae with 8 to 16 segments are typical of many gnats.

Now to elminate them you need to understand their life cycle. Adult females lay about 200 transparent eggs (each about one millimeter long) into moist soil. After about one week the larvae head to the top of the soil or anywhere they can get out and hatch. The adults now feed and survive only long enought to reproduce.


The first way to get rid of them: The environmentally friendly and all that way. Good for moderate infestation.

1. Potting media - Gnats are attracted to PEAT so decreasing peat in the soil will make it less attractive for breeding.
2. Watering - Avoid excessive watering especially when adults area active. Don't allow puddles of water to build up. I have seen the larvae growing in droplets of water on the top of a seedling container.
3. Top dress with rock or even beter washed sand as gnats will just walk around through larger rocks.
4. Sticky traps placed near the soil level will catch the adults GREATLY reducing their numbers.
5. Hypoaspis miles - Do work but are more effective in garden beds than pots. They take about 2 weeks to build up. The only problem with Hypos is they are slower than gnats + larvae and find it difficult to completely kill them off.

The above will work but requires constant monitoring.

The second way: The 100% effective way but can require chems. Required for bad infestation or if you just cant get rid of the last few.

Follow steps 1 to 4 as above.

The problems with most commercial insecticides is they won't kill off ALL the larvae and due to the gnats highly evolved breeding cycle they will just build up again and they do it very quickly.

Obtain one of the following:

1. A Larvae killing bacteria called, Bacillus thuringiensis Var israeliensis. Must be Var Israeliensis ! is found under brand name VectoBac (this one can be hard to obtain)

2. A chemical called Triflumuron found in a few products available to Australia will kill the larvae and not effect anything else including Hypos. I have personally used this product with 100% sucess. It can be found....seek and you shal find.

Apply either of the above to the soil and the larvae will die off. Once the adults are reduced the problem is gone.

In addition,

There is a snail and slug killer by Bayer that contains Methiocarb, a good larvae killer, I'm not sure what its called. A friend of mine swears by if for killing off the larvae. I was too scared to use it on my plants due to the other chems in it but could be worth looking into.

Quote :

Bacillus thuringiensis Var israeliensis

Pretty sure this is in that Gnatrol product too.

Quote :

A chemical called Triflumuron found in a few products available to Australia

Any hints as to what products contain Triflumuron?

And lastly, great write up HN, I stickied it because this is a serious issue/problem that many growers are faced with. Be good to eliminate the gnats from all our collections. Stupid gnats!

I wonder if one of the chems used to kill larvae in turf could be used. they are pretty common at the hardware store... I will have a look and might give it a try. and keep an eye out for Triflumuron

I posted up a link in Propagation section which has an article mentioning how to avoid fungus gnats.

Probably much harder than planted in beds but it looks like the layer of grit on the surface in my pots is working. I certainly haven't seen any adults scurrying about.

I have a lot of plants other than cacti, so I can't use the "keep 'em dry" strategy, and so far I haven't been able to find a source of decent grit. I have plenty of gnats though...

Most people's problem with 'chemicals' seems to be with the toxicity aspect. I haven't seen it mentioned here, but I am wondering - has anyone tried derris dust on the blighters? It's fairly innocuous to humans, and being a plant product it should be biodegrable too.

I've used it for lice elimination on chooks with great results, and I'm keen to try it on my green babies if there's no major reason not to - assuming too that it will work.

Anyone know if it's an option?

WoodDragon,

A mate of mine swears by Diatomaceous earth for gnats and many other pests even ants. He just sprinkles a little over the plant and then top dresses the soil/rocks with it. Aparently DE works by absorbing the lipids from the waxy layer of outer skin on the pest. This causes dehydration and they die. He just sent me a little to try so I wil let you know how it works.

DE is 100% organic and completely safe for mammals. Only problem is it can be hard to find and there are two types you need the agricultural, non heated or treated grade.

Greenharves sell is one of the only places that sell it. I have seen it for sale at some specialist cacti places.

http://www.greenharvest.com.au/pestcontrol/diatomaceous_earth_prod.html

The DE is supposed to be great stuff! It definitely is difficult to find. Also just a note, make sure to wear a proper dustmask when using as the fine particles can make their way into the lungs.

Apart from that, I have heard many people waxing lyrical about this stuff being a miracle media. Am yet to try it, but am very very keen.

Trig it's supposed to also be good for mites and preventing diseases on slow growing cacti. Sounds like its worth a try. As I said a mate fo mine has used it for ages and loves it.

Yeah I've really been meaning to get some along with some replacement plastic for the greenhouse, will do so after Xmas. Pretty broke right now. Damn Xmas, nah it's pretty good because I know one of the pressies I will be getting is the Ariocarpus etcetera book

What size DE is recommended for cacti mixes HN, do you know? There are few sizes available.

Cheers.

I've been nosing around for ages for the sort of DE that is recommended, but I've had no luck down here. I used to use it in labs years ago for filtration - had no idea then that I'd have another use for it!

But oh, the idea that I could slice the blighters as they sneak under the surface of the soil...!

I'll keep looking I guess...

Trig,

As far as I know for insect control it must be the fine powder.

Does anyone know how many times it is possible to water a DE-treated pot before the DE becomes mixed in the general soil matrix to the point where its abrasive qualities are lost?

No idea, sounds interesting. I thought it was more the adsorption of lipids that killed the pests. I will have to read up on it a bit !!

I've always taken for granted that the abrasive properties were the primary method of inflicting death to exoskeletoned organisms.

Somewhere in my fused, rerouted, and generally overtaxed memory banks I recall a source that said that DE-silicas that do not have (or have lost) the sharp projections, grown by the original diatom organisms, are not nearly as effective in their lethal action.

Hmmm... definitely time for a defrag!