Parasites and pathogens are ubiquitous, and pose a threat to all living organisms. Investment in resistance mechanisms to fight parasite challenge can be costly, often resulting in trade-offs with other life-history traits. Host nutrition can alter the availability of resources to invest in resistance mechanisms and influence host-parasite interactions and their outcomes. I investigated the impact of nutrition on disease resistance in the cabbage looper, Trichoplusia ni. I assessed the role of dietary macronutrients on the expression of fitness costs exhibited by a T. ni strain that has evolved resistance to the bacterial pathogen, Bacillus thuringiensis (Bt). Reduced pupal weight and growth rate, which are fitness costs associated with Bt-resistance, resulted from reduced food intake rather than impaired macronutrient utilization. When given a choice, Bt-resistant T. ni self-composed a higher ratio of protein to carbohydrate (P:C ratio) than Bt-susceptible T. ni, allowing males to eliminate a fitness cost (reduced pupal weight), but not females. Next, I investigated the interaction between host nutrition and another key environmental factor, temperature, on the interaction between T. ni and two species of baculoviruses differing in host range (TnSNPV, narrow range; AcMNPV, broad range). Optimal performance of T. ni shifted to higher P:C ratios when challenged by either virus as survival increased with dietary protein content. This effect was strongly affected by temperature when challenged by AcMNPV but not TnSNPV. Virus performance was also differentially affected by the host’s environmental condition, such that AcMNPV had a broader peak of optimal performance (combined measure of host mortality and virus production) across environmental conditions than TnSNPV. Lastly, I examined the impact of nutritional stress on the ability of Bt-challenged T. ni to prime the immune system of their offspring. If parental T. ni experienced only nutritional stress or Bt-challenge, they transferred nutritional stress tolerance or immune priming to their offspring respectively. However, as surviving each stressor is costly, when experienced simultaneously a trade-off was observed where only immune priming was transferred. This study highlights the important influence of host nutrition on host resistance to pathogens, costs associated with resistance, and pathogen virulence and growth.
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Thesis advisor: Cory, Jenny
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