The Effects of the Symbiont Rickettsia on the Interactions Between a Whitefly Pest (Bemisia Tabaci) and a General Fungal Pathogen (Beauveria Bassiana)

Abstract

Some intracellular symbionts of insects confer host protection from a variety of bacterial, fungal and viral pathogens as well as from predators and parasitoids. Within the cryptic species complex of whiteflies known collectively as Bemisia tabaci is a cosmopolitan invasive agricultural pest, which is commonly infected with the symbiont Rickettsia sp. nr. bellii. Rickettsia swept rapidly through southwestern USA whitefly populations of the MEAM1 species and has been associated in a genotype-dependent manner with increased whitefly fitness and female biased sex ratios. Here we sought to determine whether Rickettsia in MEAM1 might have a defensive role against the general entomopathogenic fungus, Beauvaria bassiana. Nymphs from two lines of whitefly, each with Rickettsia positive (R+) and negative (R-) sublines were exposed to different doses of B. bassiana. The results provided evidence of protection by Rickettsia in one genetic line (MAC1) but not in the other (MAC2). In a third experiment, females of the four sublines were each outcrossed for two generations with males from an outbred whitefly culture, derived from the field within the year, and F2 nymphs from these four new sublines were exposed to the fungus. In this experiment, Rickettsia was protective in both MAC1-O and MAC2-O lines. Taken together, our results suggest the symbiont Rickettsia can confer protection against a generalist entomopathogenic fungus, B. bassiana, and that this protection is conditional on host genotype. To our knowledge, this is the first record of an insect symbiont conferring protection against a generalized and commercially available biological control agent. Bemisia tabaci MEAM1 is a global pest of warm temperate and tropical agriculture, and the prevalence of Rickettsia in many populations of this species could limit the predictability or efficacy of fungal pathogens as a potential management tool.