Protein-biased diets enhance immune responses but increase fungal susceptibility in desert locusts.

Nutritional composition has the potential to play a critical role in immune function and pathogen susceptibility. While food restriction generally suppresses immunity, the effects of macronutrient balance on host defense remain unclear. Here, we investigated how dietary protein-to-carbohydrate (p:c) ratios influence immune function and survival in the desert locust (Schistocerca gregaria) following exposure to the entomopathogenic fungus Metarhizium robertsii. Locusts were maintained on diets with varying p:c ratios, and their survival, pathogen load, growth rate, food consumption and immune responses were assessed. Locusts consuming a protein-biased diet exhibited heightened phenoloxidase but suffered higher mortality and greater fungal sporulation post-infection. These results show that increased immune investment does not necessarily translate to improved survival. Importantly, our findings have direct implications for locust biocontrol strategies using Metarhizium fungi. Given the slow mode of action of fungal pathogens, increasing plant protein content via nitrogen fertilization could accelerate host mortality while enhancing fungal sporulation and facilitating pathogen transmission within locust populations. This study underscores the role of macronutrient balance in shaping host-pathogen interactions and offers a novel approach to improving the efficacy of fungal biopesticides.