Susceptibility to disease across developmental stages: Examining the effects of an entomopathogen on a grasshopper (orthoptera: Acrididae) pest

Abstract

An important but under studied component of grasshopper ecology is how pathogens affect grasshopper population dynamics. However, insect population responses to disease may depend upon age demographics which vary temporally. In a field experiment, we varied grasshopper [Camnula pellucida (Scudder)] developmental stage (3rd instar, 4th instar, 5 th instar, and adults) and fungal pathogen exposure (lab and field) and measured grasshopper mortality and survival. Lab exposed grasshoppers were directly inoculated with the pathogen, while field exposed grasshoppers were passively exposed to naturally occurring spores. Disease increased grasshopper mortality rates and decreased overall survival. However, this effect varied with grasshopper developmental stage and mode of pathogen exposure. Adults were far less susceptible to fungal infection than nymphs. Disease mortality was 52% higher in lab exposed grasshoppers compared to individuals exposed to a natural fungal epizootic in the field. Lab exposure decreased 3rd instar survival rates more than exposure to the pathogen in the field. In contrast, grasshopper survival was invariant with disease exposure for 4th and 5th instars due to peak levels of naturally occurring fungal spores. By August 2010, the field epizootic was declining and food availability became the most important determinant of adult grasshopper survival. Therefore, age demographics may need to be considered when predicting how grasshopper populations will respond to disease.