Compost flies

[womble]

I left a couple of melon plants with a friend while I was on holiday and after picking them up, have found them to be absolutely crawling with those little compost flies.
It's not her fault I must say, I had a problem on a plant next to them on the windowsill, but dealt with them and they are all mostly gone, but they must have laid eggs in the melons compost.

There are honestly loads and loads of them, I've never seen it so bad before.

I want to plant out the melons into the greenhouse, but I'm not sure if I need to get rid of the flies first or not, I don't want to infect the whole greenhouse, on the other hand, I only get these flies inside, so maybe they would die off in a greenhouse, or maybe a GH is enough like inside, so they would go forth and multiply.

You can see my problem.

The flies would take a while to deal with inside and I'd like to get the melons outside asap, so tell me what to do please.

[Bigmallly]

Is it worth re-potting in fresh compost?

[Shadylane]

Fungus gnats. I had loads on the overwintering chillies. They're definitely worse inside than out. Someone suggested putting a layer of vermiculite or sand on the soil to stop freshly hatched ones from emerging.

[Adrastea]

I think what you have are Fungus Gnats. I have had trouble with them indoors in the past. Even when I have repotted they have come back. The problem will still occur in the greenhouse as it is a warm, moist environment which these gnats seem to like.

They lay eggs in the compost which hatch out to become tiny black headed maggots. These maggots feed on organic matter in the compost and sometimes they feed on the young tender roots of your plant. The best way to kill them off is to water on an insecticide. Because you are growing melons you could try an organic spray containing plant fatty acids. Homebase sell one. I've just used it on my Bougainvillea, excessive spray does damage the leaves.

It is really important to use a sterile good quality potting compost. Also hanging yellow sticky traps may help. You could try scraping the top off the compost where any eggs will be. Hope any of this helps.

[womble]

Repotting isn't really an option.

I've just hoovered up what I could, I'll keep them isolated for the moment and go full on to kill them for the next few days.
Thanks for the advice.