Fungus gnats

[CarterBeatsTheDevil]

I've got a collection of cacti and succulents which I absolutely love and am very proud of. My husband has a rubber plant which developed fungus gnats (different room). He didn't want to treat them and now all the plants in my room (about 14) have fungus gnats. They all got them at about the same time so I didn't have time to isolate one in particular.

I know lots of people don't necessarily bother to treat them but when you have 14 infested plants that's a lot of tiny flies knocking around.

I've scattered diatomaceous earth in them and dried them out, but I couldn't leave them without water any longer (they'd been left for 4 weeks) and having watered them a week ago two of them are already showing signs of infestation again. They were very well drained. Is this just eggs from the last batch? Do I just keep going with the diatomaceous earth after every water?

The 3 worst affected are outside atm having been blasted with fly killer.

I've also ordered more sticky traps and some more nematodes to continue treating next week.

I have no idea where they've come from.. I strongly suspect a new bag of potting compost.

Apparently they’re really common in commercial composts 😔

I used nematodes and got rid of them relatively quickly. You can get nematode ‘pearls’ now which keep for a lot longer and are really easy to apply, you just shake them on and water them in. I’ve got Mr Fothergill’s NemaKnights which have worked well, but the first time I used nematodes it was the refridgerated pack of sandy stuff that you had to dissolve and then dilute.

I used the dissolvable powder nematodes and covered the top of each badly infected pot with a plastic bag and the numbers are greatly reduced although there are still some adults flying around the house. The bags should stop them re-laying eggs in the compost. I will do a second application next week.

Nematodes! Going to give them a go. Great advice generally. Thanks all.

Definitely don't bin anything!

Any that can be grown in water- take cuttings and start again.

Sticky yellow traps, those rocks you put down that flies stick to.

Drying out and bottom watering helped me a lot.

I'm also considering using a different potting medium instead of soil if they come back, moss or something.

I had to change all the pots on my collection (put them in plastic with holes, not straight in decorative pot).

It was a massive hassle but had to be done

They like high levels of humus. Go for a soil based compost not peat free coir or bark based,or go for a grittier compost. And a layer of grit on top.

Just thought I'd come back and say the nematodes seem to have made a big difference
I changed all the yellow stickies yesterday and have only caught a handful since..

I'm repeating the treatment at the weekend.

That's great! They've made a massive difference here too. The second round is due this week. I've only seen a couple of flies flying around now and it's nice they're not trying to fly into your mouth every time you eat!

I'm surprised they're interested in cacti as I thought these gnats only liked damp soil.

I have managed to get rid of them by using grit on top of the soil so they can't lay eggs and watering less.

I also put used matchsticks upside down in the pots as i heard they don't like the sulphur!

I'm surprised they're interested in cacti as I thought these gnats only liked damp soil. They like the fleshy roots, and I guess the mother lays her eggs just after watering and hasn't been casing the joint to see whether the soil stays moist.

What are the yellow sticky things please?

@RudolphTheRedNosedSpaniel they're just sticky traps which you stick in the soil and they catch the adults. Proper ugly tbh but they do help

Lots on Amazon.

They’re sold as whitefly traps. Rectangles of paper, sticky on both sides. Downside is they’ll catch good beasties as well.

I thought mine had gone down lately.

But l now see Mr Spider has set up home near to the plant. And I’ve just watched him catch one and wrap it up in his web.

Sort of horrible and fascinating at the same time.

We've been cultivating spiders like mad (just generally think they are good people) and I can see they're trying but they've contributed very little, I'm afraid.

However, I haven't watered any of mine for 4 weeks and the fungus gnats in my office are down to the very occasional one. I'm going to repot a couple that are in badly draining pots and bottom water the rest. Might also invest in some neem oil. I do have the yellow sticky things and they are great but they're not dealing with eggs or grubs. The diatomaceous earth also seems to help but it's so fine and gets EVERYWHERE.

I did however just find a bag of succulent compost in the utility that was housing a large number of the little fuckers. I hope they enjoy the green bin.

NB I feel a bit more confident leaving them without water for longer now given that they wouldn't normally expect to be watered much at this time of year anyway

Actually, the one where I think the infestation started is the one that drains least well, so I'm going to put the house leeks that were in that in terracotta pots with holes in the bottom.

OK, Mumsnet. I have my nematodes.

(a) Should I keep them in the fridge until I'm ready to use them?

(b) The directions say make a 4 litre solution and apply to an eighth of the area to be treated. If using on a smaller area, make the mixrure more concentrated. I'm not sure how to translate this to 15 normal sized house cacti?

(c) Can I keep the solution in the fridge once made up so that I can dose again in two weeks' time?

The directions do say that you can't overdose a plant with nematodes. Do we agree with this?

@MereDintofPandiculation it feels like this might not be your first fungus gnat rodeo - please impart any wisdom you have!

and @Houseplanter and @BooseysMom!

(I'm desperate to get going with them tonight so shamelessly @ing everyone I can think of)

Another vote for the nematodes in conjunction with yellow sticky flowers. We had a dreadful bout of them a couple of months back, and this has sorted it entirely without having to re-pot everything (which would be a nightmare with my indoor jungle LOL).

@CarterBeatsTheDevil I can't remember the actual instructions but I mixed it with the quantity it asked for in a bucket in the sink. This made my 'stock' solutions or something like that. It then said dilute further (I think) so I did some rough maths which involved scooping some of the solution out of the bucket with a plastic jug, topping it up and then watering all the plants from that.
I gave them all a good soaking and then let them drain.

Tbh I got a bit slapdash towards the end and was sloshing it about with abandon, because it said I couldn't overdo it.

I didn't really answer your questions, but no I don't think you can keep it once mixed. I had far too much and I have a lot of plants but it was worth it.

My second dose was this. Much simpler.

The key thing is to break the cycle, not let the flies get back into the soil to lay new eggs. So convince them that your soil is dry and free of humus and won't be any good for developing larvae. Hence the advice to cover soil with gravel.

You're now left with flies and the current generation of eggs/larvae.

Flies are easily got rid of with fly spray, and that further reduces the risk of a female being able to lay more eggs.

Meanwhile the larvae are munching away in the soil on any source of humus, including soft roots. You could just leave them, in which case they'll hatch into flies in due course and emerge from the soil, and that should be the end of it. Or you might prefer to protect your plants by killing them. So either shake out all the soil and pick out all the grubs, then repot in fresh soil. This is what I did. Or apply nematodes.