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Now that it is warming up in Sydney, I have noticed some tiny flies in the soil of my pots. I think they are root flies.

They seem attracted to the pots containing rhizomes the most[1][2]. These pots are in the shade and do not have mulch.

How do I get rid of these flies? I want to use what I am growing, so I need a solution that is safe later.

[1] My chilli seedlings are in the same approximate location. Most of these now have some mulch in the pots and do not seem to have many flies.

[2] My lemon grass sits in a nice sunny spot, has no mulch and is also looking OK.

Internet suggestions so far:

  1. Letting the top 2-3 inches of soil dry out.
  2. Placing brightly coloured cards coated in vasaline to lure and trap them.
  3. Placing cups of vinegar and water solution near your pots.
  4. Hot sauce water. The hotter the better.
  5. Vodka water. 1:1 ratio of vodka and water.
  6. Neem oil

Update

Home made Vaseline traps were no good. Store bought sticky traps are more successful.

Hot sauce water did not work.

Vinegar water did not work.

Drying out the soil works for control. But today was a hot day and things are wilting so I had to cut my drying short.

  • Possible duplicate of Getting rid of those little flies (gnats) without using a spray? on Sustainability SE? – THelper Dec 11 '14 at 13:19
  • I wouldn't seek to eliminate them but rather understand that if controlled, they are effectively harmless. Using any/all of the methods you describe ought to be sufficient control. They typically are hatched and lay eggs as soon as possible and then go die in one of the traps soon after cutting their life short. You won't necessarily get rid of them completely but they will slowly dwindle. – Enigma Dec 11 '14 at 16:49
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These silvery gray mobile larvae and flying adults are most likely to be fungus gnats.

As you have found successful control can be accomplished with:

  • drying out the top of the soil
  • Yellow sticky traps
  • an application of one liter of water and 5 ml of 5 bleach or chlorine, wait a few minutes, followed by more water to wash the chlorine out also works but is hazardous to newly rooted plants or if you add too much chlorine
  • systemic pesticides also work but are massive overkill for a problem that can be solved with good cultural practices
  • a soil mix using less peat moss will also help
  • Edit: Any chlorine left after pouring more water though the plant pot will be in same or less quantities than what is in chlorinated tap water
  • If I use the chlorine method will my rhizomes be safe for consumption? These are newly planted so i still have several months left before harvest. – Megasaur Nov 24 '14 at 20:55
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I've gotten them under control indoors with bowls of water to which I've added a couple drops of dish soap. The soap lowers surface tension, so that when the bugs land for a drink, they sink. Color seems to be an important factor for the species I dealt with. A yellow bottom bowl drowns dozens in the time it takes a blue bowl to attract only a couple victims.

The vinegar water trick works for the local fruit fly population, but not these insects.

  • Very interesting note about the color requirements. I wonder if yellow is indeed the best. Perhaps a green or red might be better? – Enigma Dec 11 '14 at 16:57
  • @Enigma It could even be the UV reflectance of the bowls that matters. I just know that my yellow bowl worked better than a similarly shaped blue bowl. My first thought was flower vs sky. – Wayfaring Stranger Dec 11 '14 at 17:18
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I wouldn't seek to eliminate them but rather understand that if controlled, they are effectively harmless. Using any/all of the methods you describe ought to be sufficient control. They typically are hatched and lay eggs as soon as possible and then go die in one of the traps soon after cutting their life short. You won't necessarily get rid of them completely but they will slowly dwindle.

Dry top soil in addition to sticky traps nearby will make severe cuts to the colonies livelihood. To work around the need to watering (and consequently keeping the topsoil moist), you can get a plastic rain meter and drill several holes near the base and jam it into the ground near but not into the root-ball of the plant(s). Water the plant by filling the rain meter as often as necessary.

This will also aid in killing off additional flies as they get trapped. The entirety of the top soil will remain dry and inhospitable to the flies and the plant still gets its water.

Alternatively, or in addition to, you can get a quick drying substrate like perlite and cover the top soil with a layer of that to help keep the soil drier on average.

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