Aphidius adults are small wasps, typically less than 1/8 inch long. The female wasp lays an egg in an aphid. When the egg hatches, the wasp larva feeds inside the aphid. As the larva matures the aphid is killed and becomes slightly puffy or mummified, usually turning tan or golden in color. The adult parasite chews its way out of the mummy leaving a hole. This native North American parasite occurs naturally outdoors and frequently parasitizes aphids in green houses.