Functional significance of parasitism-induced suppression of juvenile hormone esterase activity in developmentally delayed Choristoneura fumiferana larvae

The parasitic wasp Tranosema rostrale transmits a polyd-navirus (PDV) to it s host, Choristoneura fumiferana, during oviposition, Last-instar C, fumife rana larvae parasitized by T, rostrale early in the stadium fail to undergo metamorphosis, and injection of the wasp's calyx fluid (CxF; contains PDV) into healthy caterpillars induces a dose-dependent delay in initiation of metamorphosis (D, Doucet and M, Cusson, 1996, Entomol. Exp. Appl. 81, 21-30 ), In the present work, parasitization and injection of CxF (0.5 female equ ivalent) on the first day of the last stadium both prevented the rise in he molymph 20-hydroxyecdysone (20HE) titer observed between day 4 and day 7 in control and saline-injected larvae. Similarly, juvenile hormone esterase ( JHE) activity was depressed following parasitization or CxF injection, wher eas control larvae displayed a peak on day 4. However, neither parasitism n or injection of CxF on day 1 prevented the JH-producing glands from turning off during the first half of the last stadium, Likewise, low but clearly d etectable JH titers were observed in the first hours following the molt but very low titers, at or near the detection limit of our radioimmunoassay, w ere seen in both control and parasitized larvae on day 4. Prothoracic gland s showed no apparent sign of degeneration 4 days after injection of CxF but had significantly smaller cells than saline-injected larvae 7 days postinj ection, It is not clear whether this was a direct effect of T. rostrale PDV . Thus, disruption of spruce budworm metamorphosis by T. rostrale CxF invol ves depression of 20HE titers but is not associated with a measurable incre ase in the level of JH, as shown for some other host-parasitoid systems. In view of the latter observation, we put forward three hypotheses regarding the functional significance of the observed suppression of JHE activity in developmentally arrested C. fumiferana larvae. (C) 2000 Academic Press.