AcMNPV pathogenesis and developmental resistance in fifth instar Heliothis virescens

Autographa californica M nucleopolyhedrovirus carrying a lacZ reporter gene was used to study pathogenesis and developmental resistance in fifth instar Heliothis virescens. Compared to fourth instar larvae, the progression of infection proceeded much more slowly in fifth instar larvae, and developmental resistance in orally inoculated larvae was much more sudden and severe. The resistance occurred within the first 6 h of the fifth instar and was partially relieved by the optical brightener M2R. Inclusion of M2R in oral inocula not only increased mortality levels, but also increased both the percentages of insects expressing lacZ and the numbers of foci 24 h after infection. This early developmental resistance was not observed in insects infected intrahemocoelically, indicating that it was mediated by the midgut and/or the tracheal system servicing the midgut. Another less severe resistance was encountered in insects intrahemocoelically infected 36 to 48 h postmolt. This later resistance was partially relieved by methoprene indicating that it was, in part, hormonally mediated. Fifth instar insects fatally infected prior to 12 h postmolt never pupated, but pupation increasingly occurred in insects fatally infected 12 to 48 h postmolt. This pattern of larval/pupal death was consistent with viral inactivation of the ecdysone commitment peak.