Fungus gnats infestation!

This year I have been infested by Fungus gnats, they are everywhere in my greenhouse and even in the small raised bed I have outside. I suspect they came from a bad bag of compost is this the usual cause? How can I prevent this from happening next year?

I purchased some Sciarid Fly Nematodes and Sciarid Zoot Off which is hopefully arriving tomorrow, never used these products before any tips for their application?

 

This year when I planted out all of my seedings, I cleaned the pots and filled them with new soil after I had baked it to 150F . 115f kills the eggs 125 to 130f kills the larvae and 150f kills any adults hiding in there. I then stow the pot in the cellar for seed starting in the new year.

Dosing with something like "mosquito bits " works too but it takes several applications to get all generations and probably not as effective.

I will be baking about 200 liters more before placing in wheelie bins too tstow for making my soil blocks.

I used my wood oven outside for most so far but will switch to using my sunoven when the weather allows. That way my power bill doesn't go sky high.

Ive heard nematodes watered in work well but have no idea what the other is. Did they not have application instructions?

 

not received the items yet so I cannot check out the instructions, hopefully they will arrive tomorrow. Someone mentioned to use sterilised compost, any idea of where this can be purchased?

 

That's what baking does. It sterilises it.

 

It could be, but probably not - if the compost is fresh.
If you've ever seen a bit of rotting veg somewhere it'll likely have fungus gnats on it regardless of the time of year.
The gnats are everywhere and they'll soon sniff out moist, organic matter to lay their eggs.
If you've seen those little clouds of insects jerking about dancing in the air - even in the middle of winter - they're probably fungus gnats having a good time!

If the compost is left over from the previous season and/or has been stored outside, then it's quite possible that fungus gnats have laid eggs into it via the tiny holes that are in the bags.
But if it's fresh and has been stored properly, then it's much less likely.

Early in the season (march-may) when I have a propagator full of seedlings in my greenhouse there will be plenty of fungus gnats as it's warm and very humid - perfect for them.
Now, with temps in the high 20's there's barely a sign of them in my greenhouse - despite there being several bags of compost I've had open since last summer.
Later in the season (sept-nov) they'll be back until I clear the greenhouse.
They will also appear around my houseplants during spring and autumn when the environment suits them.
If the conditions suit them, there will be fungus gnats.
After 50+ years of gardening I've learned to ignore them.

 

I find the addition of either ji compost or some sharp sand to the peat free does make the problem less, also watering pots from the bottom.

 

Also, if you put a layer of gravel at least 1/2" thick on top of the compost, that'll stop most of them.

 

Also found Mosquito Bits very effective, have not had any in the house plants for over a year.
Just mix and soak in water then water in, only found it needed two applications a few weeks apart.

However keeping a greenhouse under control will be a bigger job particulalry if you have them in the nearby raised bed, which is where we would eleminate them from to start with.

They are generally harmless but as you will have noticed they can fly around your face, its because they are attracted to the co2 from your breath, apparently.