The parasites of flies commonly associated with confined livestock and poultry systems are small wasps (Hymenoptera) primarily in the genera Muscidifurax, Spalangia, and Pachycrepoideus of the family Pteromalidae (pictured right and below).

Parasite egg on surface of fly pupa within the puparium.	Fullgrown (third-instar) larva of parasite.

These  parasites generally lay one egg on a fly pupa after piercing the pupal case (puparium) with the ovipositor.

The egg develops through three larval stages, feeding on the fly pupa and destroying it.

The parasite pupates within the fly puparium and then cuts a hole and emerges. One parasite emerges from a parasitized pupa.

Parasite pupa developing into an adult.	Adult parasite emerging from puparium of the house fly.

In addition, many fly pupae are destroyed by the parasites probing through the pupal case and feeding on the exudate. Consequently, many damaged pupae fail to develop into adult flies.

A parasite generally requires about 3 weeks to complete its life cycle within the puparium (at 26°C, 78°F).

The most common species of parasites around confined-animal operations are: Muscidifurax raptor (Gerault and Sanders), Spalangia endius (Walker), S. cameroni (Perkins), S. nigroaenea (Curtis), and Pachycrepoideus vindemiae (Rondani). Parasitism of fly pupae by one or more of these species can reach as much as 40% in confined-animal facilities.

Another species, Nasonia vitripennis (Walker), is sometimes abundant. Its biology differs from other species in that many eggs are laid on a pupa and several (7–10) adult parasites develop and emerge from each puparium.

There is disagreement on the importance of this parasite in suppressing fly populations, but in cases where it is unusually  abundant it may be important.

In very wet manure, especially outdoors, another species, Urolepis rufipes (Ashmead) may be present. It may be important in some dairy and cattle feedlots where very wet conditions exist.