| Summary: | Endosymbionts play an essential role in the biology, physiology and immunity of insects. Many insects, including the whitefly <i>Bemisia tabaci</i>, are infected with the facultative endosymbiont <i>Rickettsia</i>. However, the mutualism between <i>Rickettsia</i> and its whitefly host remains unclear. This study investigated the biological and physiological benefits of <i>Rickettsia</i> infection to <i>B</i>. <i>tabaci</i>. Results revealed that infection of <i>Rickettsia</i> increased the fertility, the survival rate from nymph to adult and the number of female whiteflies. In addition, this facilitation caused a significant reduction in nymphal developmental duration but did not affect percentage rate of egg hatching. <i>Rickettsia</i> infected <i>B</i>. <i>tabaci</i> had significantly higher glycogen, soluble sugar and trehalose contents than <i>Rickettsia</i> negative <i>B</i>. <i>tabaci</i> individuals. <i>Rickettsia</i> also improved the immunity of its whitefly hosts. <i>Rickettsia</i> infested <i>B</i>. <i>tabaci</i> had lower mortality rates and higher semi-lethal concentrations (LC<sub>50</sub>) when exposed to the fungus <i>Akanthomyces attenuatus</i> and the insecticides imidacloprid and spirotetramat. The percentage of parasitism by <i>Encarsia formosa</i> was also reduced by <i>Rickettsia</i> infection. Overall, <i>Rickettsia</i> infection benefits <i>B</i>. <i>tabaci</i> by improving the nutritional composition of its host, and also protects <i>B</i>. <i>tabaci</i> by enhancing its resistance towards insecticides (imidacloprid and spirotetramat), entomopathogenic fungi (<i>A</i>. <i>attenuatus</i>) and its main parasitoid (<i>E. formosa</i>); all of which could significantly impact on current management strategies.
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