Aphid parasitized by a small wasp.  The parasitized aphid is characteristically swollen and brown.  An adult parasitic wasp will cut a round hole in the back of the parasitized aphids to emerge and parasitized other aphids.  Parasitic wasps are very small and usually not noticed. They lay their eggs on or in the body of the host insect. The immature stage hatches and feeds on the host and eventually kills it. Many harmful insects such as aphids, whiteflies, scales, leafminers and caterpillars are parasitized by wasps.