Exploring the interactions of bacterial secondary symbionts (BSS) in wheat aphids, Sitobion avenae F. with parasitoids

Bacterial secondary symbionts (BSS) are renowned for their capacity to protect their hosts against natural enemies. Sitobion avenae (F.) is recently reported for Hamiltonella defensa and Regiella insecticola infections. Among natural enemies, parasitoids are the most effective bio-control agents. Host feeding strategy has been adopted by many parasitoids to increase their longevity and fecundity. Parasitoid-host interactions are highly specialized and depend upon the environmental factors and the genotypic backgrounds of the species involved. Besides the innate defence system, BSS composition of hosts may also alter the outcome of aphid-parasitoid interactions. Genetically identical S. avenae clones possessed and lacked H. defensa and R. insecticola were used to assess host feeding and parasitism behaviour, host defence generality or specificity of these symbionts in this particular wheat aphid-symbiont system involving two parasitoids, Aphelinus abdominalis and Aphidius rhopalosiphi. A. abdominalis host feeding Reponses, towards of wheat aphids possessing and lacking BSS and genetic variations, studied at varying host densities and temperatures. In the last part of this study, H. defensa and R. insecticola were evaluated for defense against genetically different parasitoids A. abdominalis and A. rhopalosiphi for host feeding and parasitism. BSS confer resistance to wheat aphids against A. abdominalis host feeding in both choice and no choice tests at all aphid host densities and temperatures tested. A. abdominalis host feeding response was positively correlated with host densities and temperatures up to a certain limit. S. avenae survivorship decreased significantly from lower to higher aphid densities and temperatures in choice and no choice tests. The most striking result was A. abdominalis host preference in feeding S. avenae without BSS in choice tests. Genetic or clonal preference in host feeding was also observed when A. abdominalis were exposed to genetically different S. avenae clones in all experiments. Wheat aphid-symbiont bond for its outcome was not affected by variable host densities and temperatures. H. defensa conferred a general defence against both parasitoids but R. insecticola was significantly resistant against host feeding of A. abdominalis while it didn’t provide defence to S. avenae against A. rhopalosiphi. A. abdominalis development in S. avenae was negatively affected by both of these bacterial symbionts but A. rhopalosiphi got negative impact only from H. defensa. Parasitoids were not involved in horizontal transmission of H. defensa and R. insecticola among S. avenae populations. These fitness advantages confer to their host aphids by these endosymbiotic bacteria expound the adaptive significance of their heritable and successful spread over many insect populations especially in aphids.