Submitted to: Archives of Insect Biochemistry and Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/25/1997
Publication Date: N/A
Citation: Interpretive Summary: Cotesia congregata is a braconid wasp which induces developmental arrest in its last instar host, Manduca sexta. The mechanism(s) of action by which the parasitoid manipulates host ecdysteroid titers to favor its own development and emergence from the host are unknown. In these investigations, we report that the parasitoid exerts control at two levels: one by interfering with the hormone inhibitory signals that prevent the host's molting glands from producing ecdysteroid at inappropriate times, and second, by contributing to the small ecdysteroid peak that accompanies the parasitoid's molt to the last instar and concomitant emergence from the host. These findings provide impetus to the development of a biorational alternative for insect control, namely to kill insect pests by capitalizing on the parasitoid's ability to remove inhibiting influences and thus increase ecdysteroid levels at developmentally inappropriate times.
Technical Abstract: Although 5th (last) instar parasitized Manduca sexta larvae undergo developmental arrest and do not wander, they exhibit a small hemolymph ecdysteroid peak (300-400 pg/ul) which begins one day prior to the parasitoid's molt to the 3rd (last) instar and concomitant emergence from the host. Ecdysteroids present in this peak were 20-hydroxyecdysone,20,26-dihydroxyecdysone and one or more very polar ecdysteroids, as well as small amounts of 26- hydroxyecdysone and ecdysone. In parasitized day-1 and -2 5th instars ligated just behind the 1st abdominal proleg, hemolymph ecdysteroid levels increased in both anterior and posterior portions (100-500 pg/ul), while in unparasitized larvae, hormone levels only increased in the anterior portion (100-350 pg/ul). Thus, the ecdysteroid peak observed in host 5th instars was probably produced, at least in part, by the parasitoids. It may serve to promote Cotesia congregata's molt from the second to the third instar and/or to facilitate parasitoid emergence from the host. In parasitized day-1 and -2 5th instars ligated between the last thoracic and 1st abdominal segments, hemolymph ecdysteroid titers reached much higher levels in the anterior portion (no parasitoids present) than in the posterior portion. Therefore, it appears that parasitization neutralizes the host's ability to maintain its normal hemolymph ecdysteroid levels and, in a separate action, the parasitoid manipulates the ecdysteroid-producing machinery so that hemolymph levels are maintained at the 200-400 pg/ul characteristic of day 3-4 hosts.