How to Control Sewer Flies in Orchids

Puttering amid your prized orchids on a peaceful morning, you look down and realize that there are tiny flies crawling and hopping on the planting medium. Commonly called sewer flies, these tiny flies are actually fungus gnats. Fungus gnats are a nuisance but generally do not harm the orchids. The larvae feed on the fungi growing in the moist and decaying orchid medium. If the infestation is severe, however, the larvae may attack the roots of seedling orchids. Rather than use insecticides in the house, the simplest solution is to repot your orchids.

Orchid planting mix
Yellow, sticky aphid whitefly traps
Scissors
Bacillus thuringiensis israelensis treatment

Instructions

- 1
  Repot your orchid, using fresh orchid medium. Generally, a chunky charcoal and coconut medium or inorganic perlite mixture discourages fungus gnats and larvae.
- 2
  Remove any decaying roots or leaves when repotting your orchid. The larvae feed on decaying organic matter, including the orchid.
- 3
  Wash the orchid pot with hot, soapy water to kill any larvae lurking in the cracks and crevices. Allow to dry before adding new medium and repotting the orchid.
- 4
  Cut the yellow, sticky traps into strips and tuck them amid the orchid roots. The adult fungus gnats are captured when they hop or crawl across the trap.
- 5
  Allow the orchid medium to dry out between waterings. Fungus gnats are a sign that you are watering too often; the medium is not drying out. To provide a higher humidity, set the orchid pots on gravel or marbles in a shallow bowl. Fill with water but do not allow the water to touch the bottom of the orchid pot.
- 6
  Drench the orchid medium with a bacterial treatment containing Bacillus thuringiensis israelensis if repotting does not work. It may require several treatments before you successfully eradicate the fungus gnats.

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