Symbiont-conferred reproduction and fitness benefits can favour their host occurrence.

Double infections of and are frequent in natural populations of , a polyphagous mite species that has been a dominant species in China since 2009. However, little is known about the causes and ecological importance of such coexistences. In this study, we established strains with different infection types and then inferred the impact of the two endosymbionts on host reproduction and fitness. Double infection induced cytoplasmic incompatibility, which was demonstrated by reduction in egg hatchability of incompatible crosses. However, doubly infected females produced more eggs relative to other strains. and did not affect host survival, whereas doubly infected females and males developed faster than other strains. Such reproduction and fitness benefits provided by double infections may be associated with the lower densities of each symbiont, and the quantitative results also confirmed competition between and in doubly infected females. These symbiont-conferred beneficial effects maintain stable prevalence of the symbionts and also help drive outbreaks in combination with other environmental factors.