[strath76]

You could use a sphagnum slurry to grow your plants in. This simply involves putting a heap of sphagnum moss into the trays so that the larva are crowded out. This also leads to a pleasant side effect which is more sphagnum. This is a relatively cheap and definately safe method and provides you with more sphagnum over time.

Another method that is definately riskier is to use Kerosene on the water. I haven't tried this but it is used here in some rain water tanks to stop larvae. As kerosene is lighter/less dense than water it floats on the surface and provides a physical barrier. I am thinking that you would need to ensure that the water always remained above the drain hole of teh pots as the kerosene would definately harm the plants if it got into the soil.