Mushroom pests: gnats and flies around a grow kit
Small flies are drawn to kits. Find and treat the source in your home, put sticky traps near the kit, and keep any spray off the mushrooms. Penn State says sciarid larvae feed on mycelium.
Which flies you are probably seeing
North Spore says to find the source of gnats in your home first, such as houseplants, a compost bin or fruit left out.
Penn State Extension covers the two fly groups that matter on commercial farms, and the same insects can turn up in a home grow:
- Sciarid flies. The larvae feed on mycelium. The adults do not feed on mushrooms, but they carry mites, nematodes and mold spores.
- Phorid flies. More numerous than sciarids, but less damaging. Penn State says females are attracted by the odor of growing mycelium.
Penn State's pages are written for commercial growers. A home kit is a very different scale, and the source does not say how bad a problem is for a single kit, so treat this as general background.
What North Spore suggests for a kit
- Find and treat the source in your home first.
- For a repellent, mix a few drops of dish soap, 1 tablespoon of vinegar and 1 tablespoon of baking soda per cup of water, and spray it on the outside of the kit.
- Do not spray the mixture on the mushrooms.
- Sticky traps or plug-in traps help.
Outdoors and with straw
Cornell says straw logs have to be fruited indoors, because bugs lay larvae in them if they are grown outside. See growing mushrooms outdoors for the outdoor methods.
If something other than flies is growing on the block, use mycelium vs mold. Other kit problems are in troubleshooting, and the overall method is in how to grow mushrooms at home.
Back to How to grow mushrooms at home
Common questions
Why do I have gnats around my mushroom kit?
North Spore says to find the source in your home first, such as houseplants, compost or fruit. Penn State says female phorid flies are attracted by the odor of growing mycelium.
Can I spray something on the kit to keep gnats off?
North Spore suggests a few drops of dish soap, 1 tablespoon of vinegar and 1 tablespoon of baking soda per cup of water, sprayed on the outside of the kit. It says not to spray it on the mushrooms.
Are sciarid larvae harmful to mushrooms?
Penn State Extension says sciarid larvae feed on mycelium. The adults do not feed on mushrooms but carry mites, nematodes and mold spores.
Do traps help?
North Spore says sticky traps or plug-in traps help.
Sources
- Spray & Grow Kit FAQ, North Spore. Checked November 11, 2026.
- Pests and Diseases, Penn State Extension. Checked November 11, 2026.
- Four Methods of Commercial Cultivation, Cornell Small Farms Program. Checked November 11, 2026.
How this page is checked
Cedrick Reese combined North Spore's home-grower advice with Penn State and Cornell extension material, which is written for commercial farms. The page says where a source's scale differs from a kitchen kit. We do not give pesticide advice here, because the sources for it cover licensed farm use, not homes.