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Pathogenesis of Autographa californica M nucleopolyhedrovirus in fifth instar Spodoptera frugiperda

Department of Plant and Microbial Biology, University of California, 251 Koshland Hall, Berkeley, CA 94720-3102, USA

Correspondence
Loy Volkman
lvolkman{at}nature.berkeley.edu

We have characterized infection and pathogenesis of an Autographa californica M nucleopolyhedrovirus recombinant, AcMNPV-hsp70/lacZ, carrying the lacZ reporter gene, in penultimate (fifth) instar Spodoptera frugiperda. Bioassays revealed that while <0·1 p.f.u. of budded virus was required to generate an LD₅₀ by intrahaemocoelic injection, approximately 6000 occlusions were required orally to achieve the same mortality in newly moulted fifth instar (5⁰) larvae. In pathogenesis experiments, 78 % of the 5⁰ larvae inoculated orally with 6000 occlusions of AcMNPV-hsp70/lacZ were LacZ-positive at 8 h post-inoculation (p.i.) and 50 % had LacZ signals in tracheal cells indicating that in these larvae infection had been transmitted from the midgut to secondary target cells. At 24 h p.i., maximum numbers of midgut and midgut-associated tracheal foci were observed (mean of 35 foci per infected larva), and 88 % of the larvae were LacZ-positive. The extremely low foci-per-occlusion ratio (0·006) indicated that successful infection of midgut cells was the primary barrier to fatal infection. A second barrier involved the loss of infected tracheal cells associated with the midgut. At 24 h p.i., 88 % of the inoculated larvae had a systemic infection, but in bioassays only 51 % succumbed to polyhedrosis disease. The absence of melanized tracheal cells in the insects throughout the time-course suggested that the larvae that cleared their infections (38 %) did so by a mechanism other than a classical immune response.

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