Host dynamics determine strength of fungal pathogen control of an insect pest

Host dynamics play a pivotal role in many human and wildlife diseases. While both host density levels and developmental stages are known to affect host-pathogen interactions, they are often studied separately and rarely examined in the field. An excellent insect host–pathogen model system to study disease dynamics under field conditions is that of entomopathogens and grasshoppers. I conducted a manipulative field experiment using microcosms (cages) in western Montana to examine the effects of fungal pathogen exposure, host density, and host developmental stage on fungal disease mortality of Entomophthora grylli in the pest grasshopper Camnula pellucida. This 2 x 2 x 3 design included disease exposure (2 levels: infectious cadaver present/absent) as one factor, host density as the second factor (2 levels: ten/twenty grasshoppers), and host developmental stage as the third factor (3 levels: 3rd instar, 4th and 5th instars, and adults). Pathogen control of grasshopper populations varied across all treatment factors. The effect of disease exposure (F1,36=3.909, p<0.0001), density (F1,36=6.028, p=0.019), and developmental stage (F2,36=8.027, p=0.001) were significant. The disease exposure x density interaction (F1,36=0.6.216, p=0.017) and the disease exposure X developmental stage interaction (F2,36=6.251, p=0.005) were significant. Early instars were highly susceptible to disease, but starting densities had no effect on disease mortality. Disease induced mortality in late instars exhibited density dependence. Adult treatments exhibited low rates of diseased induced mortality. My data show that fungal pathogen regulation of grasshopper populations can vary greatly according to host dynamics (developmental stage and density).