Cellular- and symbiont-mediated defenses in Bactrocera dorsalis (Hendel) and Ceratitis cosyra (Walker) against their native and introduced parasitoids

Abstract

The use of parasitoids as pest biological control agents has been adopted globally. Nonetheless, the success of parasitoid-oriented biological control programs is dependent on the virulence of the parasitoid, a phenomenon that is influenced by several host (pest) and parasitoid-associated factors. Common among the host factors are host innate (i.e., cellular, and humoral) immune defences and host-associated bacterial symbionts. Host innate immunity, through regulating encapsulation and melanisation of the parasitoid egg, has been shown to influence host-parasitoid dynamics. Additionally, bacterial symbionts through immunomodulation, competition for nutrients, and releasing toxins, affect the development and fitness of the parasitoids. However, in the oriental fruit fly, Bactrocera dorsalis (Hendel) and the marula fruit fly, Ceratitis cosyra (Walker) (both Diptera: Tephritidae), common pests of fruticulture, the effect of host immunity on the outcome of their interaction with parasitoid wasps is unknown. Moreover, several B. dorsalis-associated symbionts are known to affect the development, reproduction, pesticide resistance, and susceptibility of this pest to entomopathogenic fungi. Nonetheless, the effect of larvae-associated gut symbionts on the outcome of the interaction between B. dorsalis and its associated parasitoid wasps is unknown. The egg-prepupal parasitoid, Fopius arisanus (Sonan) and the larval-pre-pupal parasitoid, Diachasmimorpha longicaudata (Ashmead) (both Hymenoptera: Braconidae) and are the most common parasitoid wasps used as biological control agents of B. dorsalis, globally. Additionally, since its invasion and detection in Africa, some native parasitoids, such as the larval-prepupal parasitoid, Psyttalia cosyrae (Wilkinson) (Hymenoptera: Braconidae) has been investigated as a potential biological control agent of this pest and its native counterpart, C. cosyra. Diachasmimorpha longicaudata and F. arisanus successfully develop in B. dorsalis and C. cosyra whereas P. cosyrae develops in C. cosyra but not in B. dorsalis. This is suggestive of possible differences in defence mechanisms of B. dorsalis and C. cosyra against these parasitoids, a phenomenon that could be attributed to host intrinsic factors e.g., cellular immunity and host-associated microbes that enable the pests to defend themselves against some parasitoids and not others. Moreover, the differences in wasp performance could be due to variations in parasitoid virulence. This thesis investigated the cellular immune responses of C. cosyra and B. dorsalis to parasitoids, the impact of parasitisation on B. dorsalis gut microbiota, as well as the effects of B. dorsalis-associated larval gut symbionts on parasitoid development and their consequences for parasitoid offspring fitness. To evaluate the role of cellular immune defences on the outcome of host-parasitoid interactions in B. dorsalis and C. cosyra, I investigated the melanotic encapsulation responses, total and differential hemocyte counts, hemocyte viability, and hemocyte spreading ability in the fruit flies after parasitisation by D. longicaudata and P. cosyrae. The adult fly emergence rate post-parasitisation was also recorded. I found stronger immune responses in B. dorsalis, mediated by a higher haemocyte load enabled this pest to mount effective melanotic-encapsulation responses against D. longicaudata and P. cosyrae than in C. cosyra. Furthermore, D. longicaudata was more efficient than P. cosyrae at suppressing host-immune defences, especially by reducing hemocyte spreading and the proportion of viable hemocytes in both host fruit flies. This was corroborated by the low encapsulation rates of this parasitoid and the consequential higher number of emerging D. longicaudata adults compared to P. cosyrae. These results reveal that D. longicaudata is a more efficient parasitoid wasp and can be used as a biological control agent of C. cosyra and B. dorsalis. Using next generation amplicon sequencing, parasitoid-mediated perturbations on the B. dorsalis gut microbiota were investigated in response to parasitisation by D. longicaudata and the native counterpart, P. cosyrae. My results showed that, parasitisation shaped the composition, abundance, and diversity of the bacterial and fungal communities of B. dorsalis. Parasitisation by P. cosyrae had no effect on the gut microbial diversity of B. dorsalis larvae. On the other hand, D. longicaudata induced dysbiosis in the guts of B. dorsalis larvae and increased the abundance of pathogenic bacteria genera such as Stenotrophomonas, Serratia and MorganelIa. Diversity and abundance of the fungi community of B. dorsalis were negatively affected by parasitisation by both parasitoid species. Furthermore, I investigated the impact of some B. dorsalis gut symbionts on the acceptability of B. dorsalis (axenic, symbiotic, or the bacteria mono-associated lines) by parasitoids, adult parasitoid emergence, and the fitness of parasitoid offsprings. I found that Lactococcus lactis, a common gut symbiont of this pest, increased the number of F. arisanus and D. longicaudata emerging from B. dorsalis. This bacterium also increased the fitness of the F1 parasitoid offspring. In contrast, Providencia alcalifaciens, reduced the proportion of emerging parasitoid wasps but increased F1 wasp fitness parameters such as size and fecundity. The findings of my thesis contribute to our understanding of the underlying physiological factors influencing parasitoid performance and their implications for the management of the frugivorous pest, B. dorsalis. This thesis has also identified a candidate symbiont, L. lactis that can be incorporated as a probiotic during mass rearing of the potent parasitoids, D. longicaudata and F. arisanus. This could potentially increase the number and fitness of the mass-reared parasitoids hence contributing to the development of a sustainable biological control programme against B. dorsalis.