Mechanical transmission enhances fungal pathogen control of a grasshopper pest, Camnula pellucida

Mounting scientific evidence indicates that pathogens have important implications for host populations. However, restricted dispersal and sensitivity to abiotic conditions often limits the extent passive transmission (wind, water, soil) of diseases impacts host populations. While it is well established that vectors increase disease incidence in host populations, little field work has experimentally examined whether mechanical transmission (the physical transport of a pathogen by a non-susceptible organism), can influence a pathogen’s ability to disperse and limit host populations. I conducted a manipulative field experiment using mesocosms (cages) in western Montana to examine the effects of scavengers (ants: Formica fusca) on a fungal pathogen-grasshopper pest system commonly found in western grasslands and rangelands. I implemented a 2 x 3 design which consisted of scavenger abundance (present, absent) and fungal pathogen (Entomophora macleodii) presence (resting spores, conidia, absent). Host populations (clearwinged grasshopper, Camnula pellucida) within these cages were monitored every other day. Ants increased grasshopper disease mortality by 58% (F_1,20 = 5.1234, P = 0.035) which led to greater disease reductions of grasshopper numbers. Grasshopper survival was significantly reduced by ants in resting spore exposed treatments (Breslow-Gehan = 6.380, df  = 1, P = 0.012). Ants also reduced grasshopper numbers in conidia exposed treatments, but this trend was not significant (Breslow-Gehan = 0.668, df  = 1, P = 0.414). This study indicates that mechanical transmission can result in greater reductions of host numbers than passive transmission alone.  My results also highlight the potential of ants to vector biocontrol agents to rangeland pest insects.