Synacra pauperi is a naturally occurring parasite of fungus gnats (Bradysia spp.) that is found in greenhouses. Adults are approximately the same size as fungus gnats (1/8 inch long). You will see a noticeable constriction between the head and thorax and between the thorax and abdomen. The abdomen tapers to a sharp tip. Antennae are beaded and elbowed. Arrows point to examples in this photo.
Females insert their eggs into the fungus gnat larvae. However, only one parasitic wasp emerges from each pupae. Fungus gnat larvae live until they pupate, then die, after which the parastic wasp pupates.
Fact sheets:
Biological Control of Fungus Gnats, University of Connecticut
Fungus Gnats and Shore Flies, University of Massachusetts
Fungus Gnats, University of Connecticut