Manipulation of fifth-instar host ( Manduca sexta) ecdysteroid levels by the parasitoid wasp Cotesia congregata

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/ μl) 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/ μl), while in unparasitized larvae, hormone levels only increased in the anterior portion (100–350 pg/ μl). 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 (500–3500 pg/ μl) in the anterior portion (no parasitoids present) than in the posterior portion (150–450 pg/ μl). Therefore, it appears that the parasitoid's regulation of hemolymph ecdysteroid titers occurs at two levels. First, parasitization neutralizes the host's ability to maintain its normal hemolymph ecdysteroid levels. Second, in a separate action, the parasitoid manipulates the ecdysteroid-producing machinery so that hemolymph levels are maintained at the 200–400 pg/ μl characteristic of day 3–4 hosts. This is the first report of a parasitoid's ability to interfere with the normal inhibitory mechanisms which prevent prothoracic gland production of ecdysteroid at inappropriate periods of insect growth and development.