Writing Cross-posted from themyceliummentor.substack.com
What does a mushroom fly infestation look like?
Spotting the Sneaky Signs
I just noticed a problem in one of my grow tents. These mosquito-like bugs crawling around a few of my oyster mushroom blocks.
If you’ve been growing oyster mushrooms and suddenly noticed tiny black flies buzzing around or “worms” (actually larvae) munching on your blocks, you’re likely dealing with fungus gnats—aka sciarid flies or mushroom midges. Check out image 1.
Let’s break this problem down step by step so we can all keep our grows thriving.
What Are These Pesky Gnats? They’re small black flies (about the size of a fruit fly) with long antennae and grey wings. The real troublemakers are their larvae—white or orange maggots that hatch from eggs laid in your substrate. They love oyster mushrooms in particular. https://en.wikipedia.org/wiki/Sciaridae
Where Do They Come From? These guys sneak in from outside through doors, vents, or even on your tools/clothes. Once inside, they breed like crazy in damp spots, old substrate, or overwatered blocks.
The Damage They Cause. Larvae chow down on mycelium and baby mushrooms, leaving your oysters brown, leathery, holey, or withered. I’ve found completely beautiful flush of mushrooms, but inside is a brown sludge where they are breeding. Mycelium that is dealing with these gnats turn a browm-orange-yellow colour (see image 2).
Yields can drop big time, and they might spread mold or bacteria. Adults just add to the mess by contaminating everything.
How to Get Rid of Them (and Prevent Future Invasions)
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Spot Them Early: Set up yellow sticky traps or UV light traps in your grow space—they’ll catch adults and alert you to issues.
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Clean House: Sanitize everything! Pasteurize your substrate well (heat it to kill eggs), remove spent blocks far away, and seal up entry points with screens. Leave no stone unturned! They can be hiding in small creases where organic matter might collect.
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Environmental Tweaks: Lower humidity a bit if you can (aim under 80%), boost airflow with fans, and don’t overwater. Dry conditions slow them down.
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Deal with Infested Blocks: Toss all blocks you find midges in. Trust me, it’s not worth the hassle of trying to recover them. Better yet, purge your whole grow ASAP. This has happened to me about once every 3-ish years. I’ve found taking the loss and start fresh has always been the best course of action.
If/when this happens to you, don’t panic. Just take action and take the loss. It’s just part of learning mycology.
This is a cross post from my free Skool community. I teach people how to grow mushrooms for free. My goal is to support people feed themselves, their families and to teach youth how to work with fungi.