How do I get rid of this plague of compost flies?

Hello,

I have a fair selection of plants around the house, and while I'm far from being an expert, they do generally seem in pretty good shape.

However, I seem to have a plague of small black flies that live in and around the plants. I was told that these are compost flies, and like warm damp conditions, such as the soil of house plants. Unfortunately, they are a serious pest in the house as they get everywhere, and I would like to get rid of them. The plants don't seem bothered by them, but the occupants of the house sure are!

I tried various things, including spraying with baby Bio bug killer designed for such pests, as well as pouring very soapy water into the compost. Apparently this latter method is supposed to kill the flies without killing the plant.

In both cases, there was a temporary reduction in the number of flies, but only for a day or two. Sooner or later, they were back in force.

Anyone any suggestions as to what I can do? I don't want to get rid of the plants, but I can't live with all these flies. I don't see how I can keep the plants any cooler, as they are all indoor plants, and in a centrally heated house, they are bound to be in a warm environment (they'd probably die if it were cold anyway). I also don't see how I can keep them any less damp, as I only water the plants once a week (except for a fairly large Coleus, which droops if it isn't watered twice a week). I certainly don't over-water them.

Thanks in advance for any help or advice you can give.

They are probably fungus gnats. I'd start by checking on your watering, because they are typically only a problem if you're keeping your plants a little too wet. I know you don't think you're overwatering, but I suspect your plants are staying wet too long (potting soil may be too moisture retentive). The excess moisture will be worse for your plants in the long run than the fungus gnats. Second, you can catch the adult flies with yellow sticky traps, and to take care of the larvae get a product like Mosquito Dunks that has Bt (Bacillus thuringiensis) in it and use that when you water.

Hello,

Thanks for the reply. I just had a look at the bag of potting mixture I used, and it says that it has high water retention "to reduce the time spent watering" which sounds like it could be the problem. I assumed (obviously wrongly) that if I made sure to empty out any ater that drained through into the dish below the pot, then I would be OK. I never thought about how much water stayed in the sol itself.

Is there anything I can do, other than repot all the plants, presumably removing ALL the old soil in the process? Apart from the time this would take, I'm worried about damaging the plants and their roots.

Thanks again

P.S. I managed to get a few pics of the flies, which I'll upload in case it helps.

The best thing would be to repot in different potting mix--if your plants are staying wet enough that the fungus gnats are thriving then that amount of moisture will likely hurt your plants eventually, so in my mind it's well worth the time. As far as root damage, you will disturb the roots a little in the process but that's not a killer issue either--all plants are going to need to be repotted from time to time so it's just something you deal with. You just need to shake off as much of the old soil as possible but if there's a little left it's not the end of the world. Just use the Bt when you water to get any fungus gnat larvae that might have still been in whatever old soil you didn't get rid of.

Your other option which will not be as good for the plants and isn't guaranteed to help with the fungus gnats since the soil is so moisture retentive, but you can just let the plants dry out more in between waterings. If you combine that with the Bt and yellow sticky traps you may be able to control the gnats, and your plants will certainly be happier than they would if you do nothing. But I'd still recommend repotting them in better draining soil as your best solution.

Hello,

Thanks for the reply. Judging by the feedback I've had by asking around, it seems clear that my main two problems are overly retentive soil and too much water. I'm going to cut back on the watering, and try repotting as much as I can in a more suitable soil. The ones that are not practical to be repotted can just have the top 2" of the old stuff scraped off and covered with the new, which should help until I can repo them.

How do you suggest using the Mosquito Dunks? I just had a quick search for them, and they look like tablets that are intended to float in ponds and the like. How would you use them with house plants?

Thanks again