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Stratégies de réussite parasitaire du parasitoïde larvaire Therophilus javanus et de ses potentialités d’utilisation en lutte biologique contre Maruca vitrata le lépidoptère foreur de gousse de niébé Vigna unguiculata en Afrique de l’Ouest

Abstract : Maruca vitrata Fabricius (Lepidoptera: Crambidae) is one of the most important insect pests in West Africa causing severe damage to cowpea (Vigna unguiculata L. Walp. (Fabales: Fabaceae)). The pest alone can cause up to 80 % yield losses. Damage is done by caterpillars on flower buds, flowers and developing pods. To develop the biological control of M. vitrata in West Africa, a collaboration has been established between WorldVeg and IITA that resulted in the identification of one promising parasitoid species namely Therophilus javanus (Bhat & Gupta, 1977) (Hymenoptera: Braconidae). Because of a dearth of data concerning T. javanus biology, the present work was initiated to assess the suitability of T. javanus as a classical biological control agent against M. vitrata in West Africa.In order to help for decision making regarding the use of T. javanus as a biological control agent against the pod borer, my thesis focused on some biological parameters as egg production capacity in females, immature development, the impact of host stage on the adult life cycle and the ability of adult female to localize M. vitrata on infested host plant organs based on olfactory stimuli. I’ve also investigated the morphology, ultrastructural organization and the transcriptome of the venom gland in females, and designed a PCR method for detection of M. vitrata caterpillars parasitized by T. javanus.Our study has demonstrated that T. javanus is a synovigenic species that mature eggs gradually after emergence and that egg production in progeny is influenced by the size or instar of the caterpillar host at parasitism. T. javanus is a koinobiont endoparasitoid that has three larval instars: the first and the second instars are completed inside the host whereas the third instar is achieved outside the host. Development time and fecundity were influenced by the size or instar of the caterpillar at the moment of parasitism whereas mother longevity and progeny sex ratio were not influenced. The females explored different parts of the cowpea plant but invested more time for searching on the buds. Female attraction by M. vitrata-damaged host plants odors was impacted by the plants species. Odors released from M. vitrata-infested host plant parts were discriminated from non-infested parts in selected plants, cowpea and Tephrosia platycarpa Guill & Perr (Fabales: Fabaceae), respectively. Finally we have shown that the venom gland produces proteins with similarities with venom proteins from other hymenoptera, in particular with the endoparasitoid Microplitis demolitor Wilkinson (Hymenoptera: Braconidae). However, the highly transcribed sequences were related to venom proteins in the ectoparasitoid Ampulex compressa (Fabr.) (Hymenoptera: Ampulicidae). Primers designed based on the sequence of the most expressed venom protein in T. javanus allowed to discriminate adult T. javanus from other adult hymenoptera parasitoids. Similarly, parasitized M. vitrata caterpillar could be distinguished from non parasitized M. vitrata caterpillar.These findings are discussed in the frame of improving mass rearing of the parasitoid in laboratory, and optimizing release strategies, but also of developing an approach for investigation of the parasitoid establishment. This thesis represents the first documentation devoted to the physiology and bioecology of T. javanus. However, some questions still remain to be addressed in order to decide on the possibility of using T. javanus as biological control agent against M. vitrata in West Africa.
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Djibril Aboubakar Souna. Stratégies de réussite parasitaire du parasitoïde larvaire Therophilus javanus et de ses potentialités d’utilisation en lutte biologique contre Maruca vitrata le lépidoptère foreur de gousse de niébé Vigna unguiculata en Afrique de l’Ouest. Parasitologie. Université Montpellier; Université d'Abomey-Calavi (Bénin), 2018. Français. ⟨NNT : 2018MONTG057⟩. ⟨tel-02010558⟩

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