Beat Fungus Gnats: Sticky Traps, BTI, Bottom-Watering

If you’re reading this, you either already have a gnat problem or you’re determined to avoid one. Good news: fungus gnats are one of those pests that respond well to a thoughtful, combined approach. I’ll walk you through exactly what they are, why they’re so persistent, and how I’ve successfully eliminated them from my apartment plant jungle using sticky traps, bottom-watering, BTI drenches, and a few soil habits that actually work. This isn’t a spray-and-pray guide — it’s a practical plan rooted in how fungus gnats live and reproduce, and what you can change without tossing every plant out.

Why fungus gnats are so stubborn (and why that’s useful)

Fungus gnats have a compact, efficient life cycle: egg, larva, pupa, adult — and it’s over in about three to four weeks under normal indoor conditions.^[1] The adults live one to two weeks and don’t bite or transmit diseases; their chief crime is laying hundreds of eggs in moist potting mix. The larvae are the real problem. They live in the top inch or two of soil, feeding on fungi, decaying organic matter, and sometimes tender root hairs. That feeding can stunt plants and, with heavy infestations, cause wilting and root damage.^[2]

What makes them stubborn is simply biology. The eggs and pupae can hide in soil, and adults will keep flying around, laying more eggs if they find damp, organic-rich soil. But this life cycle also gives us targets: stop the adults from breeding, kill the larvae where they live, and change the soil environment so eggs can’t survive.

The best wins come from combining tactics: trap the adults, dry the surface soil, and treat the larvae — not just one or two half-measures.

How to confirm you actually have fungus gnats

Sometimes people confuse fruit flies or drain flies for fungus gnats. Here are signs that point to fungus gnats specifically:

• Small, slender, dark flies about 1/8 inch long that hop or erratically fly near plant soil.
• Larvae: tiny, translucent, worm-like grubs with black heads in the top inch of soil. I once scooped a little potting mix and found these curling around a root — a telltale sign.
• Increased flying after watering. Adults appear most when the soil surface is moist.
• Plant symptoms: slowed growth, yellowing leaves, and root damage in severe cases.^[3]

If you see both adults and larvae, assume an active infestation and plan for an integrated response.

Fast triage: what to do immediately

When I first find gnats, I run a quick, three-step triage: trap adults, let soil dry, and isolate the worst plants.

1. Put down yellow sticky traps. Place at least one near each affected plant and one more by the window or shelf where many flies congregate. The traps cut the adult population quickl

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References

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Footnotes

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