Power bars: Mormon crickets get immunity boost from eating grasshoppers

Authors: Srygley, Robert; Branson, David - Dave

Submitted to: Insects
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/7/2023
Publication Date: 11/10/2023
Citation: Srygley, R.B., Branson, D.H. 2023. Power bars: Mormon crickets get immunity boost from eating grasshoppers. Insects. 14(11):868. https://doi.org/10.3390/insects14110868.
DOI: https://doi.org/10.3390/insects14110868

Interpretive Summary: Mormon crickets are omnivorous, feeding on plants, fungi, and insects, including one another. Insects contain more protein than plants, and so prey availability might be an important factor determining the protein in Mormon cricket diets. Some grasshoppers co-occur with Mormon crickets and feed on the same hostplants, but little is known about their interactions. We hypothesized that if Mormon crickets feed on grasshoppers, then the Mormon crickets’ needs for alternative protein sources would decline when grasshoppers were more numerous. In addition, because Mormon crickets with less dietary protein had less immunity, we hypothesized that greater grasshopper density would enhance Mormon cricket immunity. In a field setting, we varied numbers of Mormon crickets from 0 to 20 and numbers of grasshoppers Melanoplus borealis from 0 to 45 m-2 in 68 1-m2 cages. After one month, we measured Mormon cricket dietary preferences and immune activity. As predicted, we found that protein consumption from the alternative source declined as grasshopper density increased, and immunocompetence increased with grasshopper availability. In addition, plant nitrogen declined with increasing insect density, reinforcing the importance of predation by Mormon crickets to meet their protein needs. These results have implications for management. Mormon crickets might be an agent regulating grasshopper populations. In addition, grasshopper abundance might be an indicator of Mormon cricket immunity, particularly to fungal infection.

Technical Abstract: In addition to feeding on plants, Mormon crickets Anabrus simplex predate on invertebrates, including one another, which effectively drives their migration. Carnivory derives from lack of dietary protein, with Mormon crickets deprived of protein having less phenoloxidase (PO) available to combat foreign invaders, such as fungal pathogens. Because Mormon crickets commonly occur with grasshoppers that feed on the same plants, we investigated interactions between grasshoppers and Mormon crickets, and hypothesized that if Mormon crickets are predatory on grasshoppers, grasshopper abundance would influence protein available to Mormon crickets and their immunity. In a field setting, we varied densities of Mormon crickets (0, 10, or 20 per cage) and grasshoppers Melanoplus borealis (0, 15, 30, or 45) in 68 1-m2 cages. After one month, we measured Mormon cricket dietary preferences and PO activity. As predicted, artificial diet consumption shifted away from protein as grasshopper density increased, and immunocompetence, as measured by PO activity, also increased with grasshopper availability. While nitrogen availability in the vegetation decreased with increasing insect density, predation became an important source of protein for Mormon crickets. Grasshoppers can be an important source of dietary protein for Mormon crickets, with grasshopper availability affecting Mormon cricket immunity to disease.