Choristoneura fumiferana entomopoxvirus prevents metamorphosis and modulates juvenile hormone and ecdysteroid titers

Larvae of the spruce budworm, Choristoneura fumiferana, infected with C. fu miferana entomopoxvirus (CfEPV) continue to feed and grow without undergoin g metamorphosis and die as moribund larvae. The lethal dose (LD50) and leth al time (LT50) values for fourth instar larvae are 2.4 spheroids and 25.2 d ays, respectively. One hundred percent of the control fourth instar larvae, which were fed water instead of virus, pupated by 18 days post feeding (PF ). Only 30% of the larvae that were fed the LD50 dose and none of the larva e that were fed the LD50 dose pupated by 18 days PF. Of the control larvae, 95% became adults by 24 days PF, whereas in the treated group only 2% of l arvae that were fed the LD50 dose and none of the larvae that were fed the LD95 dose became adults by 24 days PF. Some of the virus-treated larvae die d as either larval/pupal or pupal/adult intermediates. These phenotypic eff ects were similar to the larval/pupal and pupal/adult intermediates, result ing from treating larvae with juvenile hormone (JH) or its analogs, which s uggests that EPV may cause such abnormalities by modulating JH and/or ecdys teroid titers. In untreated sixth instar larvae the JH titer decreased to l ow levels by 24 h after ecdysis and remained low throughout larval life. EP V-fed sixth instar larvae had 2112 pg/ml on day 0, 477 pg/ml on day 1 and 8 75 pg/ml on day 8 of the sixth instar. Control larvae contained 860 ng of e cdysteroids per mi hemolymph on day 8 of the sixth instar, whereas EPV-trea ted larvae of the same age (30 days PF) had only 107 ng of ecdysteroids per mi of hemolymph. Thus, EPV infection results in increased JH titer and dec reased ecdysteroid titer. Northern hybridization analysis was performed usi ng RNA isolated from control and EPV-fed larvae and cDNA probes for (i) juv enile hormone esterase (JHE), which is JH inducible, (ii) Choristoneura hor mone receptor 3 (CHR3), which is ecdysteroid inducible, and (iii) larval sp ecific diapause associated protein 1 (DAP1), whose expression is larval spe cific. EPV-treated larvae showed higher levels of JHE and DAP1 mRNA and low er levels of CHR3 mRNA, indicating that they had higher levels of JH and lo wer levels of ecdysteroids. Thus, our data show that EPV prevents metamorph osis by modulating ecdysteroid and JH levels. (C) 2000 Elsevier Science Ltd . All rights reserved.