Comparative pathogenesis of Autographa californica M nuclear polyhedrosis virus in larvae of Trichoplusia ni and Heliothis virescens.

We compared early viral pathogenesis and dose-mortality relationships for larvae of two highly susceptible hosts, Trichoplusia ni and Heliothis virescens, using a construct of AcMNPV containing the lacZ reporter gene. Larvae were inoculated either as newly molted fourth instars (4(0)) or 15 hr after the molt (4(15)). In 4(0)-inoculated larvae, first lacZ expression was detected in the midgut epithelium of T. ni at 4 hr postinoculation (hpi) compared to 18 hpi in H. virescens, and systemic infections were initiated from tracheole cells servicing the midgut epithelia beginning at 12 and 20 hpi, respectively. The longer viral tenure within the midgut and the slower progression of systemic infections within H. virescens ultimately contributed to its longer time to death. For 4(0)-inoculated H. virescens, proportions of lacZ-expressing larvae increased from 18 hpi until the onset of the molt to the fifth instar at 36 hpi; at this time point, the proportion of signaling insects equaled the final larval mortality. Viral infections within the midgut epithelium of H. virescens were lost during the molt to the fifth instar. Dose-mortality relationships suggested that the peritrophic membrane provided little protection from AcMNPV infections for either species.