Elevated hemolymph protein and branched-chain amino acids are associated with fungal Metarhizium growth and host mortality in the desert locust, Schistocerca gregaria.

Dietary macronutrient balance is a major determinant of infection outcomes, yet the mechanisms linking host nutrition to pathogen success remain poorly understood. In the desert locust, Schistocerca gregaria, protein-biased diets enhance several markers of immune function but paradoxically increase susceptibility to infection by the entomopathogenic fungus Metarhizium robertsii. We hypothesized that dietary protein may benefit the pathogen by increasing access to host nutritional resources, and predicted that protein-biased diets would increase hemolymph concentrations of total protein and branched-chain amino acids, nutrients known to support Metarhizium growth, and that infection would cause declines in these compounds. We also hypothesized that infected locusts would behaviorally compensate by shifting nutrient intake toward a more resistant, carbohydrate-biased intake target. To test these predictions, we examined how dietary protein-to-carbohydrate balance influenced hemolymph nutrient reserves and nutrient intake behavior following fungal infection. Protein-biased diets increased total hemolymph protein and concentrations of 8 out of 12 amino acids measured, but did not affect hemolymph glucose or trehalose concentrations. Infection caused declines in glucose, protein, and the three branched chain amino acids and tyrosine. Infected locusts did not alter nutrient intake behavior. Together, these findings are consistent with the hypothesis that protein-biased diets accelerate mortality during fungal infection by creating a more favorable nutritional environment for the pathogen.