Bio-based control strategies of Bruchus rufimanus in faba bean crop

This thesis broaches the promotion of faba bean cultivation (Vicia faba L., Fabaceae) for food uses in Wallonia and the challenge posed by the broad bean weevil, Bruchus rufimanus Boheman 1833 (Coleoptera : Chrysomelidae). Faced with no effective control methods against this pest, biocontrol strategies and alternatives to pesticides were investigated. Five approaches were adressed, including the improvement of the pest bio-ecological knowledge, the identification of optimal varieties for seed valorization in food industries, the evaluation of most effective semiochemical traps, and the development of specific control methods based on entomopathogenic fungi and RNA interference.
Chapter I provided an overview of faba bean cultivation and the challenges posed by Bruchus rufimanus (BBW) infestations in Europe. The chapter reviews the biology and chemical ecology of BBW, identifies knowledge gaps, and explores potential improvements to semiochemical-based approaches and alternative control methods. This chapter lays the groundwork for the subsequent research areas of the thesis.
Chapters II focused on the ecology and the biology of BBW in Wallonia. Field monitoring studies were conducted to gather comprehensive data about adult infestations, addressing the lack of region-specific information. The diversity of bruchids and their parasitoids in faba bean crops are described, including their spatial distribution and the damage they cause to seeds. Bruchus rufimanus was identified as the predominant bruchid species, along with four larval parasitoids: Triaspis luteipes, T. thoracica, Pteromalus sequester, and P. fasciatus. Infestation rates and emergence timing of BBW varied across different bioclimatic areas. Chapter III provides a methodological description of rearing Bruchus rufimanus and models the influence of temperature on the pest's embryonic and post-embryonic development.
Chapter IV evaluated faba bean varieties and the combined influence of climate on crop productivity, seed quality, and Bruchus spp. infestations. The aim was to identify promising varieties for local protein sourcing in the food industry. Fourteen varieties were assessed over two seasons, taking into consideration parameters such as protein production and bruchid infestations. A ranking methodology based on principal component analyses is presented, along with the varieties most suitable for the Walloon agro-climatic context. Factors promoting varietal resilience in European contexts were also identified.
Chapter V focused on the semiochemical control of BBW, aiming to identify efficient traps for capturing BBWs, assess the traps' impact on BBW sex ratios, evaluate collateral effects on crop benefits, and examine the influence of crop stage on trap captures. White pan traps combined with floral kairomones were found to be the most effective. Crop phenology strongly influenced trap attractiveness, and no significant sex ratio trends were observed. However, semiochemical traps had a significant impact on beneficial insects, requiring further adaptation to minimize collateral effects, particularly for threatened species. Recommendations are provided for minimizing impacts on beneficial entomofauna.
Chapter VI investigated in laboratory potential new biopesticides against BBW. Firstly, five strains of entomopathogenic fungi were screened for their lethal and sublethal effects on bruchid species. B. bassiana (GHA) exhibited the highest virulence in terms of TL50. Additionally, the study explores RNA interference with C. maculatus as a biotechnological tool for bruchid control. All the necessary proteins for gene silencing and the laccase 1 gene were successfully identified. Micro-injection of laccase 1 dsRNA resulted in a significant decrease in gene expression in treated adults. Although no significant mortality was observed, further research should focus on the larval stage or explore alternative target genes to induce lethal effects. The study demonstrates successful gene silencing in a bruchid species and highlights RNAi as a potential control method.
Chapter VII summarizes the different control levers evaluated in the thesis for the development of integrated biocontrol strategies against B. rufimanus. The most suitable crop itinerary for minimizing BBW impacts is presented as well as areas for future research to further develop the main conclusions of the thesis.