Each bag contains several thousand dark fly pupae that have been parasitised in the insectory. To protect the brittle pupae, the bag also contains rice husks.
By the time you receive the fly parasites a few parasites should be visible in the bag because they will have started to hatch. If you can’t see any, then put the bag in warm place for an hour or so to encourage activity. It is not necessary to warm the bags before the parasites are released although it’s not a bad idea to check that they have started to hatch.
The fly parasites should be distributed when it’s not too windy. The idea is to distribute the flies in areas that flies select for breeding.
In general flies breed in areas where there is plenty of muck and moisture. It’s important to avoid spreading the parasites in areas where hooves might crush the fly pupae.