The Susceptibility of Bemisia tabaci Mediterranean (MED) Species to Attack by a Parasitoid Wasp Changes between Two Whitefly Strains with Different Facultative Endosymbiotic Bacteria

SIMPLE SUMMARY: Bemisia tabaci is a complex of whitefly species, of which MEAM1 and MED species are highly invasive crop pests worldwide. The ecology of B. tabaci is affected by infections of facultative endosymbiotic bacteria, which may result in fitness benefits to their hosts, thus facilitating their adaptation to new environments. In some insects, endosymbionts protect their hosts from attack by parasitoid wasps by supplementing the host’s genetic defences. Parasitoids are effective biological control agents of B. tabaci. Understanding the variation in susceptibility to parasitization between whitefly strains harbouring different endosymbiotic bacteria is of practical importance. Here, we focused on MED species using two strains of the Q1 lineage, one infected with Hamiltonella and Cardinium (BtHC), and the other with Hamiltonella and Rickettsia (BtHR). We first saw no difference between BtHC and BtHR in parasitization by Eretmocerus mundus when reared on BtHC. On the other hand, BtHC nymphs were more resistant than BtHR after E. mundus were reared on BtHR for four generations. The same was observed after seven generations. Our findings showed that host strain is a factor affecting the ability of E. mundus to parasitize B. tabaci and support the hypothesis of Cardinium as a protective symbiont of the MED species. Moreover, our findings suggest that counteradaptations to B. tabaci defence mechanisms that maximize the parasitoid fitness may be rapidly selected in E. mundus. ABSTRACT: In this study, two strains of the mitochondrial lineage Q1 of Bemisia tabaci MED species, characterized by a different complement of facultative bacterial endosymbionts, were tested for their susceptibility to be attacked by the parasitoid wasp Eretmocerus mundus, a widespread natural enemy of B. tabaci. Notably, the BtHC strain infected with Hamiltonella and Cardinium was more resistant to parasitization than the BtHR strain infected with Hamiltonella and Rickettsia. The resistant phenotype consisted of fewer nymphs successfully parasitized (containing the parasitoid mature larva or pupa) and in a lower percentage of adult wasps emerging from parasitized nymphs. Interestingly, the resistance traits were not evident when E. mundus parasitism was compared between BtHC and BtHR using parasitoids originating from a colony maintained on BtHC. However, when we moved the parasitoid colony on BtHR and tested E. mundus after it was reared on BtHR for four and seven generations, we saw then that BtHC was less susceptible to parasitization than BtHR. On the other hand, we did not detect any difference in the parasitization of the BtHR strain between the three generations of E. mundus tested. Our findings showed that host strain is a factor affecting the ability of E. mundus to parasitize B. tabaci and lay the basis for further studies aimed at disentangling the role of the facultative endosymbiont Cardinium and of the genetic background in the resistance of B. tabaci MED to parasitoid attack. Furthermore, they highlight that counteradaptations to the variation of B. tabaci defence mechanisms may be rapidly selected in E. mundus to maximize the parasitoid fitness.