Fungus Gnats, Not House Flies, from Indoor Petunias

The Real Culprit: Fungus Gnats

Those small flies emerging from your indoor petunia pot are almost certainly fungus gnats, not house flies. They breed in moist potting soil rich in organic matter (compost, peat, manure).

Why They Appear Suddenly

• Overwatering creates ideal breeding conditions.
• Eggs were likely in the potting mix from the nursery.
• Warm indoor temperatures accelerate their lifecycle.

Control Measures

1. Move the plant outside immediately—this stops indoor infestation.
2. Let soil dry thoroughly between waterings (top 1-2 inches dry).
3. Yellow sticky traps catch adults.
4. Bacillus thuringiensis israelensis (Bti) (e.g., Mosquito Bits) watered into soil kills larvae.
5. Top-dress with ½ inch sand or perlite to deter egg-laying.

Prevention

• Use sterile, well-draining potting mix.
• Water from the bottom.
• Quarantine new plants 2 weeks.

House flies are attracted to decaying food/garbage, not potting soil. Fungus gnats are a nuisance but rarely harm established plants.