Pathogenesis of Autographa californica multiple nucleopolyhedrovirus in fifth-instar Anticarsia gemmatalis larvae

We have investigated infection and pathogenesis of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) in Anticarsia gemmatalis (velvetbean caterpillar) larvae using a lacZ recombinant virus (AcMNPV-hsp70/lacZ) to track the temporal progression of infection in the midgut intestine and haemocoel. A. gemmatalis was highly resistant to fatal infection by occlusion bodies (OBs; LD(50)>5.5 x 10(5) OB) and budded virus (BV; LD(50)>3 x 10(5) BV) administered via oral and systemic routes, respectively. Orally administered occlusion-derived virus (ODV) efficiently attached and fused to midgut cells; however, high levels of infection-induced apoptosis limited infection in the midgut. Transcriptional analysis of AcMNPV genes expressed in the midgut of OB-inoculated A. gemmatalis larvae showed high levels of mRNA encoding the major capsid protein VP39 in the absence of immediate-early transactivator 1 (ie-1) expression. In the midgut, virus was efficiently transferred from infected midgut epithelial cells to nearby tracheolar cells and circulating haemocytes to initiate systemic infection in the haemocoel. However, haemocoelic BV did not efficiently disseminate infection and only cuticular epidermal cells displayed high levels of viral infection. Flow cytometry analysis of haemocytes isolated from BV-inoculated A. gemmatalis larvae showed low-level expression of the BV envelope protein GP64 on the cell surface, suggesting that A. gemmatalis haemocytes have a limited capacity for amplifying virus. These results show that AcMNPV is not an effective biological control agent for limiting crop damage caused by A. gemmatalis larvae.