Predation and immunocompetence: Grasshoppers increase Mormon cricket resistance to a fungal pathogen

Typically, herbivorous insects cope with immunity challenges by altering the amount of macronutrients (protein or carbohydrates) or plant secondary compounds they consume. Omnivores can shift their diet more broadly than herbivores to ingest animals in addition to plants, which could lead to food web consequences that have been little explored. To fill this gap, we hypothesized that omnivorous Mormon crickets would obtain a protein boost when provisioned with grasshoppers and confer a survival advantage when infected with the pathogenic fungus Beauveria bassiana . Consequently, we predicted that Mormon crickets would eat more grasshoppers when infected with the fungus than uninfected controls. We caged Mormon cricket nymphs in one of four combinations of 10 or 20 Mormon crickets and 0 or 45 grasshoppers per square meter. Three weeks later, we topically applied either B. bassiana or the oil carrier without fungus to the Mormon crickets and housed them individually outdoors. Those reared with 45 grasshoppers were provisioned with five grasshoppers, whereas those lacking grasshoppers got none. After 5–8 days, we drew haemolymph to measure total phenoloxidase (PO) and haemolymph protein. Cages were checked daily for mortality for 23 days. Significantly more fungal‐treated Mormon crickets died than control insects, but mortality was lower than expected from lab trials. Hence, B. bassiana proved to be an effective pathogen of Mormon crickets, although it grew slowly in the cool montane environment. Provisioning grasshoppers increased the circulating protein but did not significantly affect mortality outdoors. A lack of grasshoppers resulted in a decline in total PO when Mormon crickets were caged at high density, whereas there was no detectable effect of grasshopper provisioning on total PO of Mormon crickets at low density. Maintenance of the fungal‐treated individuals at 28°C for an additional 8 days indoors demonstrated that the insects were tolerating the infection in the field and those provisioned with grasshoppers had greater survivorship than those without. Although infected Mormon crickets did not consume more grasshoppers to ward off the infection and differentially affect food web dynamics, we conclude that prey abundance affected immunocompetence of a predator with nutrients from grasshoppers integrated into Mormon cricket immunity to a natural fungal control agent.