Ecology and Molecular Characterization of Neozygites tanajoae (Entomophthorales: Neozygitaceae) a fungal pathogen of the cassava green mite

For the biological control of the cassava green mite (Mononychellus tanajoa, Bondar), Brazilian strains of the entomopathogen Neozygites tanajoae were recently introduced into Africa to improve the control achieved by predatory mites. In order to monitor the establishment of the Brazilian strains two PCR primer pairs, NEOSSU_F / NEOSSU_R and 8DDC_F / 8DDC_R were tested on N. tanajoae strains collected from several locations in Brazil and from three countries in Africa, Benin, Ghana and Tanzania. The first primer pair enabled the species-specific detection of N. tanajoae, while the second differentiated the Brazilian isolates from those of other geographical origin. The results confirmed that the two primer pairs tested are suitable for the detection of N. tanajoae and the differential identification of Brazilian and African strains. PCR can therefore be used to monitor the establishment and spread of the Brazilian strains in Benin and in other African countries where they have been introduced recently in order to improve cassava green mite (CGM) control. In this work the establishment and spread of Brazilian strains of N. tanajoae were followed in a countrywide survey in Benin in which a total of 141 cassava fields were inspected. Samples of M. tanajoa suspected of being infected by N. tanajoae were found in 60 fields distributed between the coastal Southern Forest Mosaic (SFM) and the Northern Guinea Savannah (NGS) zones. PCR analysis of DNA samples extracted from samples from these fields revealed that N. tanajoae is well distributed in Benin and that Brazilian strains were effectively established and have spread throughout the country. However, the highest rates of infection due to Brazilian strains were observed in the sub-humid and humid savannah zones of the country. Identification of N. tanajoae strains using molecular tools, however, is very costly. Hence, development of alternative techniques is desirable. Therefore, the difference in biocontrol performance and in host ranges was compared to discriminate between African and Brazilian strains of N. tanajoae. The results suggest that the biocontrol potential and the host ranges bioassays are suitable for evaluating the infectiveness of N. tanajoae on cassava green mite. However, those methods were not suitable for differentiating among different origins of strains of the entomopathogenic fungus. The establishment of N. tanajoae in Benin after its introduction for the control of cassava green mite resulted in co-occurrence with the predatory mite Typhlodromalus aripo in cassava fields. However, little is known on the interaction of the two antagonists and on the biological control potential of M. tanajoa. In a series of greenhouse experiments, effects of single and combined releases of N. tanajoae and T. aripo on their respective population dynamics and on the suppression of M. tanajoa populations were evaluated. In order to complement the greenhouse experiments, laboratory experiments were conducted to evaluate the feeding, oviposition and longevity of T. aripo fed with healthy or N. tanajoae-infected M. tanajoa. The results showed that simultaneous release of T. aripo and N. tanajoae in the same cassava field may be detrimental to the biological control of the cassava green mite. It is therefore preferable to release in each area only the antagonist species known to be well adapted to the prevailing environmental conditions. In conclusion, this thesis shows that molecular techniques are the most suitable methods to detect infection of CGM by N. tanajoae and to differentiate among strains. Molecular techniques are also useful for monitoring the establishment and dispersal of introduced N. tanajoae species in the field. Furthermore, this study increased our knowledge on the performance and host ranges of the African and Brazilian strains of N. tanajoae. It also improves our understanding of the interaction between N. tanajoae and the most effective predatory mite T. aripo as both biocontrol agents are sharing the same habitats. Future research should focus on in vitro production and cryopreservation of African strains of N. tanajoae in order to develop specific primers for detecting African isolates. Furthermore, studies of the genetic diversity of N. tanajoae populations in the cassava fields using molecular techniques could strongly improve our understanding of the interaction between exotic and native fungi in the cassava ecosystem. Further investigations are required on the interaction between the predatory mite T.aripo and the pathogen N. tanajoae at a larger spatial scale and under natural conditions, and on factors affecting the loss in biocontrol potential of exotic species of N. tanajoae when introduced in their new environment.