ensign wasp, (family Evaniidae), any of a group of solitary wasps known for parasitizing the eggs of cockroaches. The wasps are so named because the small, oval abdomen is held high like an ensign, or flag. Around 400 species of this widely distributed family have been described, and many more are thought to exist. Evania appendigaster is a common North American species.
See also list of ants, bees, and wasps.
The thin-waisted body, which is typically black and somewhat spiderlike in appearance, ranges in length from about 1 to 1.5 cm (about 0.4 to 0.6 inch). Ensign wasps are beneficial to humans because all species are thought to be parasitoids of cockroaches, which are common household pests. The female wasp places its eggs in the cockroach’s egg case (oothecae), and the wasp larvae devour the cockroach eggs. As endoparasites, the wasps pupate inside the egg case and emerge as adults. Male and female adults feed on flower nectar.