I suspect you are right @Mr. Vine Eye, they will have been imported into your house in that batch of compost. I had a similar problem but on a much smaller scale and was advised by the houseplant expert at my local garden centre to use a Neudorff product called BugFree. I wouldn't normally use a pesticide but this is neo-nicotinoid free and organic with a "natural active ingredient". This eased my conscience, and it worked!
If this happens again my wife won't let me have any plants in the house at all!
Well, we don't want that happening!
The flies are attracted to moist compost and then lay their eggs in it. If the top of the compost is dry they will not be interested as it's not suitable for them. If you have a layer of something (e.g. gravel) on top of the compost that makes it difficult for them to burrow down to the moist compost so they usually go elsewhere.
I don't know what else to suggest. I only have a few houseplants and I do get them from time to time. I try and let the top of the compost dry out which gives a break from them, but they do tend to come and go. You could try SB Invigorator or Neem Oil
Billericay - Essex
Knowledge is knowing that a tomato is a fruit. Wisdom is not putting it in a fruit salad.
Are you sure the gnats didn't lay their eggs in the compost after you got it?
I was wondering if last year was a particularly good year for the gnats because of a wet spring, or similar. I've never had a problem before, but an over wintering canna was full of eggs (it turned out) and two three plants in the kitchen were crawling.
The house got so bad before Christmas that, that I used house moth spray in two or three rooms to knock out the gnats (which worked instantly). The canna is having to chance it outside. House moth spray is no long term solution, but I happened to have a can and it did get the house situ under control.
I had this problem last year as well. However, I didn't do anything at all, and the fungus gnats went away of their own accord. They're a bit annoying, but harmless, so you could just wait for the few weeks it takes before the population explosion dies down. As people have suggested, it may also be that this year won't be as bad.
@Cambridgerose12 - I'm glad that they went away quickly for you but we weren't so lucky! They didn't die down until we went all guns blazing on them. Even that took a month.
It was a plague that lasted for months and months last year.
It will be because I couldn't just let the plants dry out as we were growing plants that needed regular watering and feeding.
@Fire - they could well have, but that's why I want to treat the compost I've got before bringing it indoors again. I don't like waste and future compost is still going to have to be stored outside so it's always going to be a potential problem.
It may just be one of those things were I have to try it myself and let everyone else know!
Well I seem to be having success with this. I've been using an old oven tray that came with a turkey crown, I think, filling it with the compost mix I want to use, covering with foil and then baking it in the oven - oven set to 80 degrees and left it in there for 45 - 60 minutes. Leave to cool down to room temperature and then use.
I've been reusing the same tray and foil just for this purpose.
So far we've had no fungus gnats or other pests in the house.
I used this to treat the leftover compost from last year - and I know this compost was still riddled with fungus gnat larvae because when I used it in a propagator and lifted the lid, up they flew!
So it sorted that out and I've since used it to treat compost to use for potting house plants and any other plants that would be in the house.
It doesn't stink the house out or anything - just an earthy potato sort of sell that clears very quickly once it cools down.
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The flies are attracted to moist compost and then lay their eggs in it.
If the top of the compost is dry they will not be interested as it's not suitable for them.
If you have a layer of something (e.g. gravel) on top of the compost that makes it difficult for them to burrow down to the moist compost so they usually go elsewhere.
I don't know what else to suggest.
I only have a few houseplants and I do get them from time to time. I try and let the top of the compost dry out which gives a break from them, but they do tend to come and go.
You could try SB Invigorator or Neem Oil
Billericay - Essex
Knowledge is knowing that a tomato is a fruit.
Wisdom is not putting it in a fruit salad.
It was a plague that lasted for months and months last year.
It will be because I couldn't just let the plants dry out as we were growing plants that needed regular watering and feeding.
@Fire - they could well have, but that's why I want to treat the compost I've got before bringing it indoors again. I don't like waste and future compost is still going to have to be stored outside so it's always going to be a potential problem.
It may just be one of those things were I have to try it myself and let everyone else know!
Yep, I think that's why we pay a bit more for bespoke house-plant compost. I'm guessing the manufacturers have met this problem before.
I play with plants and soil and sometimes it's successful
I've been reusing the same tray and foil just for this purpose.
So far we've had no fungus gnats or other pests in the house.
I used this to treat the leftover compost from last year - and I know this compost was still riddled with fungus gnat larvae because when I used it in a propagator and lifted the lid, up they flew!
So it sorted that out and I've since used it to treat compost to use for potting house plants and any other plants that would be in the house.
It doesn't stink the house out or anything - just an earthy potato sort of sell that clears very quickly once it cools down.
So that works!