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AcMNPV pathogenesis and developmental resistance in fifth instar Heliothis virescens
B A Kirkpatrick1, J O Washburn, L E Volkman
1Department of Plant and Microbial Biology, University of California, Berkeley, California 94720-3102, USA.
Abstract:
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.
Insights
Fifth instar Heliothis virescens larvae show rapid, severe resistance to Autographa californica M nucleopolyhedrovirus infection, particularly in the midgut. This resistance, influenced by developmental stage and hormones, impacts viral pathogenesis and insect survival.
Area of Science:
- Entomology
- Virology
- Insect Pathology
Background:
- Autographa californica M nucleopolyhedrovirus (AcMNPV) is a significant pathogen affecting Lepidoptera.
- Understanding host-pathogen interactions and developmental resistance is crucial for biological control strategies.
Purpose of the Study:
- To investigate the pathogenesis and developmental resistance of Autographa californica M nucleopolyhedrovirus in fifth instar Heliothis virescens.
- To elucidate the mechanisms and timing of developmental resistance in insect larvae.
Main Methods:
- Utilized Autographa californica M nucleopolyhedrovirus carrying a lacZ reporter gene for infection studies.
- Administered viral inocula orally and intrahemocoelically to fifth instar Heliothis virescens larvae.
- Assessed infection progression, mortality, reporter gene expression (lacZ), and pupation rates.
- Investigated the role of optical brightener M2R and methoprene in modulating resistance.
Main Results:
- Fifth instar larvae exhibited significantly slower infection progression and more severe, sudden developmental resistance compared to fourth instars.
- Early resistance (within 6 h of fifth instar) was mediated by the midgut/tracheal system and partially relieved by M2R.
- A later, less severe resistance (36-48 h postmolt) was hormonally mediated and partially relieved by methoprene.
- Pupation occurred in insects fatally infected later in the fifth instar, suggesting viral inactivation of the ecdysone commitment peak.
Conclusions:
- Fifth instar Heliothis virescens larvae possess potent early developmental resistance to AcMNPV, primarily mediated by the midgut.
- Hormonal factors also contribute to developmental resistance later in the fifth instar.
- Viral pathogenesis and insect development are intricately linked, with AcMNPV potentially disrupting key hormonal events like the ecdysone commitment peak.
