Endosymbionts play an essential role in the biology, physiology and immunity of insects. Many insects, including the whitefly Bemisia tabaci, are infected with the facultative endosymbiont Rickettsia. However, the mutualism between Rickettsia and its whitefly host remains unclear. This study investigated the biological and physiological benefits of Rickettsia infection to B. tabaci. Results revealed that infection of Rickettsia 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. Rickettsia infected B. tabaci had significantly higher glycogen, soluble sugar and trehalose contents than Rickettsia negative B. tabaci individuals. Rickettsia also improved the immunity of its whitefly hosts. Rickettsia infested B. tabaci had lower mortality rates and higher semi-lethal concentrations (LC₅₀) when exposed to the fungus Akanthomyces attenuatus and the insecticides imidacloprid and spirotetramat. The percentage of parasitism by Encarsia formosa was also reduced by Rickettsia infection. Overall, Rickettsia infection benefits B. tabaci by improving the nutritional composition of its host, and also protects B. tabaci by enhancing its resistance towards insecticides (imidacloprid and spirotetramat), entomopathogenic fungi (A. attenuatus) and its main parasitoid (E. formosa); all of which could significantly impact on current management strategies.

Biological control is an effective method for whitefly management compared to the potential problems caused by chemical control, including environmental pollution and the development of resistance. Combined use of insect parasitoids and entomopathogenic fungi has shown high efficiency in Bemisia tabaci control. Here, we assessed the impacts of an entomopathogenic fungus, Cordyceps javanica, on the parasitism rate of a dominant whitefly parasitoid, Eretmocerus hayati, and for the first time also compared their separate and combined potential in the suppression of B. tabaci under semi-field conditions. Six conidial concentrations of C. javanica (1 × 10³, 1 × 10⁴, 1 × 10⁵, 1 × 10⁶, 1 × 10⁷ and 1 × 10⁸ conidia/mL) were used to assess its pathogenicity to the pupae and adults of E. hayati. Results showed that the mortality of E. hayati increased with higher concentrations of C. javanica, but these higher concentrations of fungus had low pathogenicity to both the E. hayati pupae (2.00–28.00% mortality) and adults (2.67–34.00% mortality) relative to their pathogenicity to B. tabaci nymphs (33.33–92.68%). Bioassay results indicated that C. javanica was harmless (LC₅₀ = 3.91 × 10¹⁰) and slightly harmful (LC₅₀ = 5.56 × 10⁹) to the pupae and adults of E. hayati respectively on the basis of IOBC criteria, and that E. hayati could parasitize all nymphal instars of B. tabaci that were pretreated with C. javanica, with its rate of parasitism being highest on second-instar nymphs (62.03%). Interestingly, the parasitoids from second and third-instar B. tabaci nymphs infected with C. javanica had progeny with increased longevity and developmental periods. Moreover, experimental data from 15 day semi-field studies indicate that combined application of C. javanica and E. hayati suppresses B. tabaci with higher efficiency than individual applications of both agents. Therefore, combined applications of C. javanica (1 × 10⁸ conidia/mL) and E. hayati is a more effective and compatible biological control strategy for management of B. tabaci than using either of them individually.

The assessment of acute toxicity to insect natural enemies is very important for insecticide selection used within integrated pest management (IPM). The acute toxicity of abamectin and imidacloprid against Encarsia formosa, a parasitoid of Bemisia tabaci, was investigated. Abamectin had a high toxicity risk to E. formosa, while imidacloprid showed a medium toxicity risk. When treated with the lethal concentration 30 (LC₃₀) of abamectin, the dwelling time of E. formosa in B. tabaci infested-plant-area (IPA) was significantly lower than in non-infested plant areas (non-IPA). In addition, the frequency of E. formosa entering into the two areas was not significantly different in the LC₁₀ and LC₃₀ treatments. Within the IPA, LC₁₀, and LC₃₀ treatments decreased the dwelling time and entering frequency of parasitoid significantly. For imidacloprid treatments, E. formosa stayed a longer time in the non-IPA than in the IPA when treated with LC_30. The frequency of E. formosa entering into the two areas was only slightly different in the LC₁, LC₁₀, and LC₃₀ treatments. Within the IPA, LC₁₀ and LC₃₀ treatments were significantly decreased in the dwelling time and the entering frequency of E. formosa. The results indicate that abamectin and imidacloprid have high or medium acute toxicity against E. formosa and a negative sublethal effect on its searching behaviour.