Plant-mediated effects on insect-virus interactions

The effects of host plants on baculoviral efficacy have been well documented; consumption of particular host plants often renders insect herbivores less susceptible to viral disease. Ingestion of cotton (Gossypium hirsutum L.) foliage in the first through third instars prior to oral viral inoculation in the fourth instar significantly reduced mortality of Heliothis virescens F. by Autographa californica nucleopolyhedrovirus (AcNPV). A construct of AcNPV that expresses lacZ under the control of the hsp70 promoter of Drosophila (AcNPV-hsp70/lacZ) was used to examine viral infection in vivo. Larvae that consumed cotton foliage in the first 8 h of the fourth instar prior to viral inoculation experienced a significant reduction in both mortality and percentage of larvae with foci of infection, compared with larvae fed artificial diet. Although cotton ingested prior to inoculation significantly reduced the percentage of larvae with midgut infection foci (indicated by lacZ signaling), there was no difference in the percentage of larvae possessing systemic tracheal infection foci, compared with larvae fed artificial diet. These results support the hypothesis that ingested cotton foliage hinders establishment of midgut infections in H. virescens, perhaps by modifying midgut physiology or by direct antagonism between virions and foliage components. This effect may not be limited to cotton, since other host plants of H. virescens (tobacco and oakleaf lettuce) also significantly reduced larval susceptibility to AcNPV when consumed prior to viral inoculation.;The peritrophic matrix (PM) of herbivorous insects lines the midgut, protecting it from damage by ingested plant material. A significantly thicker PM was observed in cotton-fed larvae compared with artificial diet-fed larvae, suggesting that this thickened PM protects the midgut cells from damage by ingested foliage and secondarily provides protection from ingested pathogens. Disruption of the PM by the metalloprotease enhancin in cotton-fed larvae restored larval susceptibility to the level of diet-fed larvae. When viral pathogenesis was examined using AcNPV-hsp70/lacZ to indicate AcNPV infection, lacZ signaling appeared in larvae fed artificial diet+enhancin significantly earlier and in a greater proportion of larvae, compared with larvae fed untreated artificial diet. Thus, the PM appears to be a barrier to baculoviral infection in cotton-fed H. virescens larvae and may be a significant factor determining pathogen success by influencing establishment of midgut infections.