Ecdysteroid titer during larval-pupal-adult development of the tobacco hornworm, Manduca sexta

The ecdysteroid titer during the fourth (penultimate) and fifth larval instars of Manduca sexta was characterized by large increases lasting approximately 24 and 60 hr, respectively, and these peaks occurred just before ecdysis. During the last larval instar, an earlier, smaller increase in the titer was also observed lasting approsimately 20 hr and is the increase presumed responsible for commitment to pupal development. After pupation, the ecdysteroid titer increases again, reaching a maximum between Days 7 and 9. At Day 10 postpupation, the titer declined dramatically and stabilized at ∼day 14. During pupal-adult development, there was no sexual dimorphism in the ecdysteroid titer. However, a difference in the ecdysteroid titer between males and females was observed after adult eclosion. Qualitative analysis of the ecdysteroid RIA activity revealed the presence of only ecdysone and 20-hydroxyecdysone at varying ratios depending on developmental stage. For the large peaks preceding larval and pupal ecdysis, 20-hydroxyecdysone was the predominant hormone, while ecdysone was the major component during pupal-adult development. The fifth instar commitment peak was comprised of approximately equal quantities of the two ecdysteroids.