Relative amount of symbionts in insect hosts changes with host-plant adaptation and insecticide resistance.

The impact of symbionts on their hosts depends on their infection density. In the current study, we investigated the effects of host plant and insecticide resistance on the relative amount of symbionts Portiera, Hamiltonella, Rickettsia, and Cardinium in the whitefly Bemisia tabaci (Gennadius) B biotype. The relative amount of symbionts in three host plant-adapted subpopulations (cucumber, Cucumis sativus L.; cabbage, Brassica oleracea L.; and cotton, Gossypium herbaceum L.) with the same genetic background and insecticide (thiamethoxam)-resistant and -susceptible subpopulations with the same genetic background were measured by quantitative polymerase chain reaction. The results showed that the cucumber population harbored more Portiera than the cabbage and cotton populations, the cabbage population harbored more Hamiltonella than the cucumber population, Hamiltonella amount did not statistically differ between the cotton and cucumber or the cotton and cabbage populations, and the cabbage population harbored more Rickettsia and Cardinium than the cucumber and cotton populations. In addition, the thiamethoxam-susceptible population harbored more Portiera and Hamiltonella than the thiamethoxam-resistant population, whereas the thiamethoxam-resistant population harbored more Rickettsia than the thiamethoxam-susceptible population. These results indicated that relative amounts of symbionts were affected significantly by host plant-adaption and insecticide resistance, and the response to host plant and insecticide differed among the symbionts.