Plant care - fungus gnats - blech!!!

So a couple of years ago, I noticed that when at work, little black bugs flew around. They weren’t especially tough to swat, and I remembered having seen something like that many years before, and it was associated with houseplants. Which I happened to have, right on my desk.

Some googling, and evidently they’re called fungus gnats. I seem to have gotten rid of them via letting the plants dry out quite thoroughly a few times (supposedly eliminates the damp-loving fungus the larvae feed on); hadn’t seen one in a year or more.

Brought the plant home. Not sure that one was an issue, as I saw no pests. Then I brought a couple new plants home… and all of a sudden we’re overrun (er, overflown?) with the damn things. In our faces (evidently they like carbon dioxide), occasionally falling into soapy water when we’re doing dishes, etc.


Googling suggests that we need to strongly avoid overwatering / standing water (ok, doing that, check), put out sticky traps (did that, check), and apply bacillus thuringiensis israeliensis (as opposed to the assorted other BT varieties out there). Bought that (granules; also suitable for adding to ponds etc. outdoors for eliminating skeeters) and applied to soil and watered - sparingly.

Nearly 2 weeks later and we seem to have as many bugs as ever. Well, probably somewhat fewer, as we’ve placed a number of sticky traps in plants and those have gotten covered in stuck dead gnats. Oh, and the BT granules? Where I sprinkled them, and wet them to “water them in”… there’s mold growing.

Someone please tell me you’ve had infestations like this and gotten rid of them! We’ve had some of these plants for years and I’d hate to have to get rid of them - one is a ficus that’s taller than I am, others are descended from plants we owned before we were married 25 years asgo. On the other hand, I’m really really tired of risking a high-protein snack every time I inhale.

Yes, I have been through this before and gotten past it. If I remember correctly the fungus gnat takes a couple weeks to go from egg to maturity. Hopefully the steps you have taken are getting rid of the adults before they lay eggs, but you still have to wait around long enough for the next generation to die off. You might also try replanting as fungus gnats are often a sign of a heavily decomposed (too much so) substrate.

Thanks - many of them are in brand-new soil, actually - I had done a lot of repotting. I’ve heard that new potting soil can often be a haven due to the organic material they put in it.

Oh well… at least the Venus flytrap plant I bought Saturday should eat well!

Have you tried spreading a layer of sand on top of the soil? Sand dries out faster than the dirt does so they don’t like it as much.

I am going through a second bout of fungus gnats myself. The first time happened a couple of years ago when I repotted using a different potting soil than usual, and I got rid of them by repotting again in the usual brand. Then a few weeks ago I decided to do some more repotting, and I bought the different brand of soil again. I had a vague recollection of having had a bad experience with that brand, but I couldn’t remember what it was, and it’s a lot cheaper, so I bought it anyway. When the gnats appeared I remembered what the bad experience was.

I’m trying to control the gnats with sticky traps, and making the soil less hospitable to them by putting a layer of vermiculite over it (got that tip from a web page on how to protect your marijuana plants from gnats, but hey, a plant’s a plant, right?). I’ll give it a couple more weeks, but if the gnats aren’t gone by then it will be time to repot again.

::snerk:: - yeah, a plant’s a plant! I guess the vermiculite is the same principal as the sand. I’ll have to see what they have at the hardware store.

When you repot, do you shake all the loose soil off or something?

I’ll have to mix in the BT granules next time I water - they’re probably not doing much good, laying on top of the soil growing mold.

I try to get all the loose soil off, but sometimes the roots are so tangled that it’s pretty near impossible. You can also rinse the soil off, but beware of clogging your drain. I had to do some amateur plumbing last time, which pretty much shot the hell out of my evening :mad: