March 22 - Cotesia congregata

Few species exhibit behavior as gruesome and horrifying as the parasitic wasp, Cotesia congregata. As the great naturalist Charles Darwin once wrote, “I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae [one group of parasitic wasps] with the express intention of their feeding within the living bodies of Caterpillars.” Cotesia congregata belongs to this superfamily of Hymenoptera and their sinister life cycle begins when the adult female oviposits up to eighty eggs, as well as a host of symbiotic viruses, into the body of a young tobacco hornworm While the viruses suppress the caterpillar’s immune responses, the wasp larvae will hatch and undergo a series of molts within the caterpillar. Feeding on the host’s bodily fluids, but carefully avoiding any damage to the vital organs, the mature larva will eventually eat through the caterpillar’s skin and find a nearby branch on which to build a cocoon. At this point the hornworm, remarkably still alive, will arch over the cocoons and vigilantly stand guard over the brood. The host will remain in this position without moving or eating until the wasps have all emerged from their cocoons. The caterpillar will sometimes go so far as to spin a protective layer of silk over the pupae as they grow and voraciously defend the cocoons from predation. The mechanism by which the Cotesia wasps control the host’s strange behavior is not fully understood. However, because a few larval Cotesia remain behind (staying within the host) during pupation, popular theory has implicated them as the primary forces dictating the host’s behavior.

See more in: Zimmer, Carl. Parasite Rex. New York: Simon & Schuster, 2001.

Contributed by Phillip Zook, Bucknell University.

Thanks to Alex Wild for the photo.