"Breakouts seem to coincide with a new bag of soil, and also certain types of soil, perhaps the peat component."
This is a real problem that has been occurring for many decades. The areas where the peat is harvested are too often over grazed and what happens is sedges and grasses, will recover in these areas before the sphagnum is allowed to grow back and dominate. Their bodies are included in the prematurely harvested peat and the flies feed on the debris of these higher plants. Their eggs and larva are bagged in the medium.
