I'm growing lots of oyster mushrooms, and now I have lots of black flies. I don't know what species they are.

How do I tell if they are fungus gnats or fruit flies ? How do I control them ?

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    Hi Jimmy! Can you please post a picture or two? One of a whole infected mushroom and a close-up of the critter? How long have they been there? Are they making holes? If you think of anything else to narrow it down, just edit it into the question. Thanks! Commented Aug 2, 2018 at 17:00
  • I'll see if I can. They look like the little black flies we get every summer in the kitchen living off fruit and veg. I haven't seen any damage from them (haven't really looked) , but they definitely are living in the bags of myceliated straw I have around the house. Commented Aug 3, 2018 at 9:21
  • So have you got actual mushrooms or just mycelium? Commented Aug 3, 2018 at 12:17
  • Sorry guys I just got some new batteries for my camera but now the bugs have all disappeared. I threw a bunch of infested stuff out thinking there would still be plenty bugs left, but there isn't. Commented Aug 11, 2018 at 11:30
  • So I missed the bugs, maybe the fact that they are seasonal tells us what they are. But I got some yellow oysters coming. I did the holes covered with micropore tape thing (which worked well for the pink oysters), but they seemed to want to grow in the space under the lid instead so I took the lid off and hey presto! BTW that's straw cold pasteurised with calcium hydroxide. cdn.pbrd.co/images/HyXF9h6.jpg Commented Aug 13, 2018 at 9:37

2 Answers 2


There are many things you can try to control fungus gnats including

  • LED blue lights ( for the non-adult stage)
  • those UV lights that attract and kill flying insects
  • sticky traps
  • Bacillus thuringiensis subspecies Israelensis (BTI)
  • pyrethrum

and best is to prevent them getting into your grow room in the first place.

  • Spiders ? I was thinking about collecting a few spiders from around the place. Commented Aug 3, 2018 at 9:18
  • OK mate, thanks for the suggestions. I'm sure now that they are fungus gnats at least, and I read that beer traps or potato can be used smallfootprintfamily.com/… Commented Jan 6, 2019 at 11:35

So, your question has two parts:

How do I tell if they are fungus gnats or fruit flies ?

You could always take a picture and put it on StackExchange! Or, you can go to the very useful https://bugguide.net and upload a picture, then wait for an expert to look at it. You'll need to set up an account, but I've had very good results and you'll be notified by email when someone responds.

But, that's for bug identification in general. For your specific case, here's a handy chart from the page https://dengarden.com/gardening/Fungus-Gnats-All-you-need-to-know:

What's the Difference Between a Fungus Gnat and a Fruit Fly?

Fungus Gnat
smaller than a fruit fly
tiny black body
attracted to moisture and decaying organic material
feeds on moist, decaying organic material found in houseplants' soil

Fruit Fly
brown or tan body
attracted to fruits and vegetables
feeds mainly on produce

How do I control them ?

The link above, https://dengarden.com/gardening/Fungus-Gnats-All-you-need-to-know, has a lot of helpful-looking tips, and you also seem to have found a good link on your own at https://www.smallfootprintfamily.com/how-to-control-fungus-gnats-organically.

  • OK so, if I put bacillus thuringiensis on my gnat infested shrummies, will they be OK to eat ? Commented Feb 3, 2020 at 19:30

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