BACKGROUD Cardinium symbiont is a maternally inherited bacterial endosymbiont and widely spreads in arthropods including Bemisia tabaci (Hemiptera: Aleyrodidae). However, the potential role of Cardinium played in the biology of their hosts is largely unknown. In two genetic lines (i.e. LS and SG lines) of B. tabaci MED, collected from different locations in China, we tested the effects of Cardinium on the performance of the host whitefly under a constant high temperature (31 °C) using the age‐stage two‐sex life table method, and explored the genes influenced by Cardinium‐infection by RNA‐sequencing. RESULTS We found that Cardinium did provide protection of B. tabaci against heat stress under 31 °C. However, there was a significant connection between Cardinium‐infection and whitefly genetic backgrounds. Performance revealed that Cardinium infection can increase the longevity of both female and male adults and oviposition periods in both lines, but it also conferred benefits of fecundity and pre‐adult period to LS line. Additionally, the population parameters such as intrinsic rate of increase (r), finite rate of increase (λ) and mean generation time (T) demonstrated that Cardinium infection conferred fitness benefits to LS line but not to SG line. Transcriptome analysis indicated that several genes related to homeostasis and metamorphosis such as ubiquitin‐related genes were highly expressed in Cardinium‐infected B. tabaci. CONCLUSION The research provided the first evidence that Cardinium can increase the thermal tolerance of whitefly, which may be associated with host genetic background.

Communities of bacteria, especially symbionts, are vital for the growth and development of insects and other arthropods, including Bemisia tabaci Mediterranean (MED), a destructive and invasive insect pest. However, the infection density patterns and influence factors of bacteria in whiteflies, which mainly include symbionts, remain largely unclear. To reveal the different density patterns of Cardinium in B. tabaci MED populations and the impacts of high temperatures on whiteflies with different Cardinium density infection patterns, 2 isofemale lines isolated from B. tabaci MED from the same geographical population of China and from B. tabaci MED collected from other countries and locations were examined using several techniques and methods, including fluorescence in situ hybridization (FISH), quantitative real‐time polymerase chain reaction (qPCR), 16S rRNA gene sequencing, and 2b‐RAD sequencing. The results showed that there were 2 different infection density patterns of Cardinium in B. tabaci MED (including 1 high‐density pattern and 1 low‐density pattern). For whiteflies with low‐density Cardinium, conventional PCR could not detect Cardinium, but the other techniques confirmed that there was a low level of Cardinium within hosts. High temperature significantly decreased the diversity of bacterial communities: the relative titer of Cardinium increased but the density of Rickettsia decreased in the isofemale line with high‐density Cardinium. However, high temperature did not influence the diversity and symbiont density in the line with low‐density Cardinium. Moreover, high temperature influenced the functions of bacterial communities in whiteflies with high‐density Cardinium but did not affect the bacterial functions in whiteflies with low‐density Cardinium. Our results provide novel insights into the complex associations between symbionts and host insects.