Understanding how diet and temperature affect survival and subsequent sporulation in a major rangeland grasshopper pest, Melanoplus sanguinipes, infected with the entomopathogenic fungus, Metarhizium robertsii

•We analyzed how Metarhizium inoculation and febrile conditions impacted intake targets in M. sanguinipies.•All treatments selected the same carbohydrate-biased IT of 1:2 protein:carbohydrate (p:c).•Grasshoppers with access to febrile conditions were able to completely rescue themselves from inoculation.•Inoculated grasshoppers in the carbohydrate-biased and protein-biased treatment groups survived longer than the balanced diet.•Post-mortem Metarhizium growth was greatest in the balanced diet treatment and minimal on grasshoppers eating the other diets. Behavioral fever is well-described in insects as an effective response to pathogens, but recent research also shows that the balance of macronutrients is important. Australian plague locusts (Chortoicetes terminifera) infected with Metarhizium acridum, a fungal entomopathogen, had longer survival by increasing carbohydrate and decreasing protein consumption. Our research tested the effects of Metarhizium robertsii (strain DWR2009) on the dietary macronutrient balance (Intake Target, IT) of the migratory grasshopper (Melanoplus sanguinipes, one of the most pestiferous grasshoppers in the United States), with and without elevated temperatures, to determine if pathogens significantly influence diet selection in this species. We also tested the effects of diet on survival under a M. robertsii inoculation. We found no significant difference in the ITs across all treatment groups; all treatments selected the same carbohydrate-biased IT of 1:2 protein:carbohydrate (p:c). In the prescribed diet experiments, inoculated grasshoppers from both the carbohydrate-biased (7p:35c) and protein-biased (35p:7c) diet treatment groups survived longer than those fed the balanced (21p:21c) diet. However, grasshoppers with access to elevated temperatures were able to completely rescue themselves from the pathogen. In correlation with our results, post-mortem Metarhizium growth was greatest on grasshoppers fed the balanced (21p:21c) diet and minimal on grasshoppers eating the other two diets. Eating the balanced 21p:21c diet either did not support the host to mount an effective immune response, provided a nutritionally optimal environment for pathogen growth, or both. Eating a protein-biased diet potentially supported an effective immune response, whereas eating a carbohydrate-biased diet starved the pathogen of protein and/or supported an immune response via different pathways.