Protein deficiency lowers resistance of Mormon crickets to the pathogenic fungus Beauveria bassiana

Authors: Srygley, Robert; Jaronski, Stefan

Submitted to: Journal of Insect Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/14/2018
Publication Date: 2/1/2018
Citation: Srygley, R.B., Jaronski, S. 2018. Protein deficiency lowers resistance of Mormon crickets to the pathogenic fungus Beauveria bassiana. Journal of Insect Physiology. 105:40-45. doi:10.1016/j.jinsphys.2018.01.00.

Interpretive Summary: Lack of nutrients can cause Mormon crickets to migrate in large bands. Mormon crickets in some bands prefer protein to sugar-rich foods and appear to be moving in search of proteins. In this study, we tested whether Mormon crickets that are protein-deprived have weaker immunity to fungal attack, and we assayed two potential mechanisms of this immunity: phenoloxidase activity and the ability to encapsulate a foreign object. Beginning in the last juvenile instar and continuing into adulthood, crickets were fed a high, intermediate, or low protein diet with correspondingly low, intermediate, or high carbohydrate proportions. Adult Mormon crickets fed high protein diets had higher prophenoloxidase titers, greater encapsulation response, and higher survivorship to Beauveria fungal infection than those on low protein diets. We replicated the study adding very high and very low protein diets to the treatments. A high protein diet increased phenoloxidase titers, and those fed the very high protein diet had more circulating prophenoloxidase. Mormon crickets fed the very low protein diet were the most susceptible to B. bassiana infection, but the more concentrated phenoloxidase and prophenoloxidase associated with the highest protein diets did not confer the greatest protection from the fungal pathogen as in the first replicate. We conclude that protein-restricted diets caused Mormon crickets to have lower phenoloxidase titers, slower encapsulation of foreign bodies, and greater mortality from B. bassiana infection than those fed high protein diets. These results support the nutrition-based dichotomy of migrating Mormon crickets: protein-deficient ones are more susceptible to pathogenic fungi whereas carbohydrate-deficient ones are more vulnerable to bacterial challenge.

Technical Abstract: Little is known about the effects of dietary macronutrients on the capacity of insects to ward off a fungal pathogen. Here we tested the hypothesis that Mormon crickets fed restricted protein diets have lower enzymatic assays of generalized immunity, slower rates of encapsulation of foreign bodies, and greater mortality from infection by Beauveria bassiana, a fungal pathogen. Beginning in the last nymphal instar, crickets were fed a high, intermediate, or low protein diet with correspondingly low, intermediate, or high carbohydrate proportions. After they eclosed to adult, we drew hemolymph, topically applied B. bassiana, maintained them on diet treatments, and measured mortality for 21 days. Mormon crickets fed high protein diets had higher prophenoloxidase titers, greater encapsulation response, and higher survivorship to Beauveria fungal infection than those on low protein diets. We replicated the study adding very high and very low protein diets to the treatments. A high protein diet increased phenoloxidase titers, and those fed the very high protein diet had more circulating prophenoloxidase. Mormon crickets fed the very low protein diet were the most susceptible to B. bassiana infection, but the more concentrated phenoloxidase and prophenoloxidase associated with the highest protein diets did not confer the greatest protection from the fungal pathogen as in the first replicate. We conclude that protein-restricted diets caused Mormon crickets to have lower phenoloxidase titers, slower encapsulation of foreign bodies, and greater mortality from B. bassiana infection than those fed high protein diets. These results support the nutrition-based dichotomy of migrating Mormon crickets, protein-deficient ones are more susceptible to pathogenic fungi whereas carbohydrate-deficient ones are more vulnerable to bacterial challenge.