Technical Abstract: 1. Mormon crickets form large migratory bands that march over rangeland in the western United States seeking salt and protein. Immune defense is particularly relevant to survival in migratory bands, but little is known about the role of nutrition in insect immunocompetence. We hypothesized that immune defenses are compromised in these migratory bands due to nutrient limitations. 2. In a migratory band in Utah, we investigated whether access to a protein relative to a carbohydrate diet would immediately reduce migratory activity, as had been shown for Mormon crickets in a previous study in Idaho, and whether the protein diet would enhance immune defense responses. 3. Radio-tracking Mormon crickets in the field, we found that locomotor activity was significantly and positively associated with body mass. Body mass-adjusted locomotor activity declined following access to a protein diet, whereas spontaneous phenoloxidase (PO) activity was enhanced by the same diet. The encapsulation response and lysozyme-like activity were directly proportional to body mass, but unaffected by the dietary treatments. Within 6 h of feeding on protein or carbohydrates, Mormon crickets exhibited measurable effects on the immune system. 4. We conclude that spontaneous PO activity was limited by dietary deficiency in a protein-limited band of Mormon crickets.