The microclimate of the breeding medium is also of major importance in determining the rate of fly development and fly abundance.

Accumulated manure and other fly-breeding media (silage, hay, feed) generate considerable heat as they ferment. Therefore, the temperature in the breeding medium is usually higher than the surrounding air (whether indoors or outdoors).

The greater the accumulation, the more the heating, and the greater the range of temperatures that occur, providing fly larvae with greater opportunities to find a suitable habitat for development.

During the winter in cold northern areas, slow development of fly larvae and longtime survival of pupae routinely occur indoors in confined-animal facilities. The same occurs outdoors in piles of manure, silage and hay.

In the summer, the rate of fly development in accumulated manure or other breeding medium is often faster than expected from weather data. Confined-animal housing systems and a concentration of manure and feeds make for an artificial environment, protected from the extremes of weather and conducive to fly breeding.

Since the confined-animal production system is an artificial creation of humans, it can be manipulated to either encourage or discourage fly breeding. Covering and protecting feeds (silage, hay bales, grain) from moisture, and minimizing fermentation    provides a less desirable habitat for fly breeding.

Management practices in this regard are major factors in determining fly abundance.