Entomopathogen biodiversity increases host mortality

In biological control communities, greater predator species richness often strengthens pest suppression. The impacts and importance of species richness in insect-killing pathogen (entomopathogen) communities, however, has received less attention. Here, we manipulated species richness within a community of three soil-dwelling entomopathogenic nematodes (Heterorhabditis megidis, Steinernema carpocapsae, Steinernema feltiae) and one fungus (Beauveria bassiana), and measured resulting effects on mortality of Colorado potato beetle (Leptinotarsa decemlineata) and wax moth (Galleria mellonella) hosts. When potato beetles were the focal host, increasing pathogen species richness led to a linear increase in host mortality. This diversity effect appeared to result primarily from the pairing of nematodes and fungus within diverse communities; these pairings always produced host mortality that exceeded predictions based on the impact of nematodes or fungus alone. We then conducted an experiment using wax moth as the focal host, because this allowed greater replication, and included two different soil types to see if changing the foraging environment altered niche differences (and thus complementarity) among pathogen species. We again found an increase in host mortality at higher pathogen richness levels due to the apparent nematode-fungus synergism, and this effect was detected across both soil types. Our findings suggest that the positive effects of species richness commonly observed among predator species may extend to pathogen communities as well, such that conserving pathogen biodiversity may carry additional benefits for biological control.