In agroecosystems, food webs are often structured from the crop, which enables the associated herbivores, including pests, to develop. For instance, monoculture in banana fields allowed the development of the banana weevil (Cosmopolites sordidus) populations. Borrowing larvae of C. sordidus cause banana plants topple over, which dampens the yield in most production areas. In this Ph.D. thesis, we attempted to disclose the structure and function of the banana food web, and particularly the trophic interactions that link banana weevil to others species. The applied perspective was to enhance population of generalist predators in order to increase the natural regulation of the pest.1. What is the effect of adding a cover crop on the predation of the pest?By using a variety of methods - stable isotopes analyses, trapping, artificial infestation of banana trees, we successfully tested the hypothesis according to which the addition of a cover crop, by enabling population of alternative preys to develop in the system, induces a change in predator diet, an increase of predator densities, and a greater predation rate on the eggs of C. sordidus.2. How is structured the food web?We combined next generation sequencing (454 technology) with the DNA barcoding concept to identify prey into gut contents of consumers. We used a chloroplastic marker (trnL) to identify the diet of herbivores, and a mitochondrial marker (CO1) for predators. This approach enabled the detection of unexpected species, the identification of the natural enemies of the pest, and the weighting of trophic interactions at the population scale.3. How can food web structure influence pest regulation?We searched structural elements (network motifs) occurring in the food webs of two banana agroecosystems (on bare soil and with cover crop), and we inferred the system functions. We detected the “bi-fan” size-4 motif, which occurred more frequently in the cover cropped food web than in random food webs. Interestingly, this motif was unbalanced for one of the two resources, illustrating the asymmetrical distribution of interaction strengths that shapes food web structure. The analysis of the position of C. sordidus within key motifs revealed its preferential interactions with other species of the community.This Ph.D. thesis emphasizes how linking innovative and complementary methods provides a comprehensive approach of the trophic functioning of the banana agroecosystem. Our results show the importance of primary resources (other than the cultivated crop) on the structure of arthropods’ food webs and on the pest regulation potential. This work also illustrates the link between the community structure and the evaluation of associated functions (i.e. pest regulation)
