Evidence That the Stilbene-Derived Optical Brightener M2R EnhancesAutographa californicaM Nucleopolyhedrovirus Infection ofTrichoplusia niandHeliothis virescensby Preventing Sloughing of Infected Midgut Epithelial Cells

Previous reports have shown that the stilbene-derived optical brightener M2R increases mortality and hastens death of larval lepidopterans orally challenged with baculoviruses. The mechanism of viral enhancement, however, has not been elucidated. We used a reporter gene recombinant ofAutographa californicaMNPV (AcMNPV-hsp70/lacZ) to investigate the enhancing effects of M2R on pathogenesis, mortality and time to death in larvae ofTrichoplusia ni,andHeliothis virescensinoculated as newly molted fourth instars (40s) and at various hours after molting (e.g., 16 h = 416). For both hosts, M2R significantly reduced the time to death by AcMNPV. In 40-inoculated larvae, M2R accelerated infection of tracheal cells, but overall the effects on pathogenesis were minor, and no significant increase in mortality over controls was observed. Among infected M2R-treated 416H. virescens,there were significantly more foci per insect compared to controls, and first lacZ expression was detected in the midgut epithelium at 6 h postinoculation (hpi). In contrast, no lacZ signals were detected in the midgut epithelia of control insects at any time point, and first lacZ expression was observed in the tracheal system at 14 hpi. When we compared virus-induced mortality inH. virescensinoculated at various times during the fourth instar, we found that M2R treatment enhanced mortality levels progressively for larvae inoculated later during the instar. Our results suggested that inT. niandH. virescens,M2R enhanced AcMNPV pathogenesis by blocking the sloughing of infected primary target cells in the midgut, thereby countering developmental resistance and increasing mortality.