Facultative Endosymbiont Serratia symbiotica Provides Fitness Benefits for Celery Aphid Semiaphis heraclei Collected from Plant Cnidium monnieri

<i>Semiaphis heraclei</i> Takahashi (Hemiptera: Aphididae) serves as a vital resource for natural enemies from functional plant <i>Cnidium monnieri</i> (L.) Cusson (Apiaceae), playing a crucial role in ecological dynamics. Endosymbionts influence the performance of their hosts. Here, we determined the communities of facultative endosymbionts in aphids from <i>Lonicera japonica</i> Thunb. (Caprifoliaceae), <i>Apium graveolens</i> L. (Apiaceae), and <i>C. monnieri</i> and assessed the performance of four aphid clones. The infection rates of <i>Serratia symbiotica</i> Moran (Gammaproteobacteria: Enterobacteriaceae) and <i>Regiella insecticola</i> Moran (Enterobacteriales: Enterobacteriaceae) reached 100%. Notably, the infection rates of <i>Spiroplasma</i> and <i>Rickettsia</i> varied across host plants. Fitness assessment revealed that aphids performed better on their natal hosts, exhibiting shorter nymphal development times and higher fecundity. <i>S. symbiotica</i> had contrasting effects on aphids based on their origin. It prolonged the development duration and decreased the intrinsic rate of increase (<i>r</i>_m), net reproductive rate (<i>R</i>₀), and finite rate of increase (λ) in aphids collected from plant <i>A. graveolens</i>. However, for aphids collected from plant <i>C. monnieri</i>, it shortened the doubling time (<i>DT</i>) and improved <i>r</i>_m, <i>R</i>₀, and λ, while prolonging the mean generation time. Our studies are the first to investigate the infection status and role of facultative endosymbionts in aphid <i>S. heraclei</i>, extending the documented effects of plant diversity to fluctuations in the infection rate, with potentially far-reaching consequences for related endosymbionts' ecosystem processes.