Many insects harbor heritable endosymbionts, whether obligatory or facultative, and the role of facultative endosymbionts in shaping the phenotype of these species has become increasingly important. However, little is known about whether micro-injected endosymbionts can have any effects on aphid clones, which was measured using various ecological parameters. We examined the effects between symbiotic treatments and the vital life history traits generated by Regiella insecticola on the life table parameters of Sitobion avenae. The results showed that R. insecticola can decrease the intrinsic rate of increase (r), the finite rate of increase (λ) and birth rate and can increase the mean generation times (T) of S. avenae clones, suggesting that R. insecticola may decelerate the normal development of the hosts. No significant differences of these parameters were observed between the examined Sitobion avenae clones, and the symbiont treatment by genotype interaction affected only the net reproduction rate R 0 , pre-adult duration and total longevity but not the other parameters. Additionally, a population projection showed that R. insecticola decelerated the growth of the S. avenae clones. The evocable effects of R. insecticola on the S. avenae clones may have significant ramifications for the control of S. avenae populations under field/natural conditions. Many insect species harbor endosymbionts, which are indispensable to their survival and common reproduction. Because the hosts and their endosymbionts share a common fate, endosymbionts can often profoundly affect the ecology and evolution of their hosts. Among them, the endosymbionts in aphids are ideal targets for studying their role in ecological fitness 1 . In nature, almost all aphid species possess the primary endosymbiont Buchnera aphidicola, which belongs to the class γ-proteobacterium. B. aphidicola synthesizes essential amino acids and other nutrients that are ingested in very small quantities from restricted diets, such as plant phloem, for their host aphids 2-4 . Moreover, aphids can also be infected with one or more non-essential secondary endosymbionts belonging to distinct bacterial lineages 1 . For example, Regiella insecticola, Hamiltonella defensa and Serratia symbiotica belong to the class γ-proteobacterium, and Rickettsia and Spiroplasma belong to the class α-proteobacterium [5][6][7][8][9][10][11][12][13] . Secondary endosymbionts do not present in all aphid individuals, suggesting they are not generally required for the survival and reproduction of aphids. However, endosymbionts can significantly affect host aphid ecological fitness and behavior. In the past few decades, secondary endosymbionts have been found to confer benefits on the pea aphid Acyrthosiphon pisum (Harris), including resistance to heat shock (S.

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