Effect of host nutrition on the outcome of mixed infections in an insect-pathogen association

Hosts are commonly exposed to multiple pathogens. Co-infecting pathogens may compete for resources, interact through the host immune system or interfere directly with each other. Moreover, resource heterogeneity can alter host investment in defence mechanisms and thus is likely to be a major force shaping interactions between pathogens and altering pathogen epidemiology. I investigated the impact of nutrition on host resistance to co-infections in an insect system, using the cabbage looper, Trichoplusia ni, and two of its pathogens, the T. ni-specific nucleopolyhedrovirus (TniSNPV), and the generalist entomopathogenic fungus, Beauveria bassiana. I first evaluated the role of resource quantity and quality on the outcome of mixed infection, by introducing T. ni larvae to artificial diets post-pathogen challenge. While neither diet quality (Protein-to-Carbohydrate ratio) nor quantity affected overall larval mortality, fungus-induced mortality did increase with increasing protein in the diet. Beauveria bassiana was the dominant pathogen in our system, negatively affecting virus-induced mortality in all mixed infections. While fungus yield increased when co-infected with TniSNPV at a low dose, virus yield did not differ in mixed infections but decreased with increasing protein. Production of virus transmission stages and virus virulence after mixed infection were influenced by complex interactions between nutritional resources, competition, and infection dose. Next, I investigated the effect of host plant on the outcome of mixed infections after pathogen challenge. Mortality was generally higher on the poorer host plant. Moreover, the order of introduction altered the outcome of co-infections, revealing synergistic, antagonistic, and neutral relationships depending on plant identity. Lastly, both pathogens can be used as microbial control agents. Hence, I explored potential benefits of combining TniSNPV and B. bassiana to manage T. ni populations in the field and asked whether crop plant identity could alter the outcome of mixed pathogen applications. Overall, larval mortality was comparable between mixed applications and TniSNPV alone. Virus-induced mortality decreased by 50% when applied with the fungus while B. bassiana was shown to be more sensitive to host plant. These results demonstrate that pathogen competition is highly dependent on the environment and could have major implications for pathogen epidemiology and disease outbreaks in wild populations.