Symbiotic sabotage: Do aphid endosymbionts influence chemical tolerance and sensitivity to entomopathogens?

The endosymbiotic bacteria found in insects have diverse effects on their host's phenotypes. These effects may benefit host fitness and be adaptive or may decrease fitness and be maladaptive. Endosymbionts that decrease fitness provide useful candidates for novel environmentally-friendly pest control methods (e.g., through microinjection) that can be used in combination with more traditional chemical control strategies. Such combined strategies require that manipulated endosymbionts do not increase the risk of pesticide resistance, which might negate the negative fitness costs of other traits. We use aphids (superfamily Aphidoidea, order Hemiptera) as a study system to examine the interspecific and intraspecific effects of a novel transinfection of the endosymbiont Rickettsiella on chemical tolerance. Prior work has demonstrated that Rickettsiella transinfections can negatively impact life history traits, but additional effects on chemical tolerance are undetermined. Our interspecific experiments demonstrated that introducing this endosymbiont (via microinjection) does not increase chemical tolerance among three aphid species (Diuraphis noxia, Myzus persicae, and Rhopalosiphum padi). Our intraspecific experiment on four clonal strains of M. persicae showed that whilst the transinfection had no effect on chemical tolerance, infected resistant strains have a higher susceptibility to the entomopathogen Beauvaria bassiana. This indicates a possible trade-off between resistance to chemicals and entomopathogens in the presence of the Rickettsiella endosymbiont. This work collectively shows that costs of Rickettsiella infections can be generally exploited to provide a sustainable aphid management strategy that works well with entomopathogens and widely used chemicals.