Guerre dans les ténèbres: alternatives biologiques pour lutter contre les taupins

Wireworms (Coleoptera : Elateridae) are common polyphagous soil-dwelling pest that feed on roots and tubers of many crops of high economical value. The use of pesticides is not always efficient because even a low density of wireworms can affect the field and be responsible for important economical damage. The withdrawal of many insecticide formulations (including neonicotinoids), leaves producers without control methods. They call for the development of efficient, cost-effective and environmentally-friendly solutions. In Chapter I, we list the existing alternatives to pesticides: they include natural enemies, biological agents and semiochemicals. We also discuss the potential of several associations of alternatives, such as the attractand-kill strategy that couple the use of microbial agents with semiochemicals. The aims pursued during the past four years of research were the development of such an attract-and-kill strategy of wireworm’s control. To do so, several sub-goals have been set : (a) developing behavioral assays on the wireworm orientation behavior (b) profiling volatile organic compounds (VOCs) released in the rhizosphere of maize (c) evaluating the role of the identified VOCs on wireworms foraging behavior (d) developing an attract-and-kill based on VOCs, associated with entomopathogenic nematodes (EPNs). In Chapter III, the main aim was the research of maize odors that could be attractive for wireworms. After field trials, we tested the hypothesis according to which different maize varieties have differential susceptibility/attraction to wireworms because they emit different VOCs from the roots. The work was focused on the VOCs quantification from the roots of two maize varieties having experienced contrasted levels of wireworm infestation in the field. We showed that the least susceptible variety released a more diverse blend of VOCs, including large amounts of hexanal, heptanal and 2,3-octenanedione. Behavioral assays showed that wireworms were strongly attracted to VOCs released by maize roots. However, when wireworms were offered both maize varieties, no preference was detected, contrasting with the field results. In Chapter IV, we investigated whether wireworms are able to use VOCs as cues to assess the suitability of maize plants that are already under attack by conspecifics. Seven compounds from plants and larvae have been identified, amongst which 2-Pentylfuran was present in high quantity, particularly in treatments where larvae were present. Behavioral assays in dual-choice olfactometer showed a significant attraction to 2- Pentylfuran. Finally, in Chapter V, we describe an attract-and-kill method using EPNs as biocontrol agent against wireworms. We first compared the efficiency of several EPN strains through classical screening laboratory assays (contact application of EPNs). Then, we developed an attract-and-kill system proposing alginate beads filled with EPNs and spiked with potato extract to larvae. The potato extract was previously tested as attractive for wireworms and easy to produce and to use in the laboratory tests. We found that the ingestion of these beads led to higher mortality rates. This method also demonstrated some signs of reduction of the wireworms metabolism. Through this work, we were able to deeper study the wireworms orientation behavior in terms of attraction to susceptible maize varieties. We showed that VOCs are part of the cues for the host plant detection. Still regarding the behavior, the larvae aggregation with conspecifics on plant roots was highlighted for the first time, opening an important research path. Finally, the development of the attract-and kill system combining VOCs with EPNs showed a long-term reduction of wireworms activity that needs to be tested in the field. The results obtained during these years of research will certainly open new paths in the study of the aggregation, the cues for the host selection and preference that could be combined to other biocontrol methods. In particular, the results open new perspectives on the development of a more efficient attract-andkill with the combination of more specific VOCs like 2-Pentylfuran and other EPNs strains. The pursuit of this research could lead to effective and innovative solution in wireworms control