Guerre dans les ténèbres: alternatives biologiques pour lutter contre les taupins | War in the darkness: biological alternatives to control wireworms

Wireworms (Coleoptera : Elateridae) are common polyphagous soil-dwelling pest that feed on roots andtubers of many crops of high economical value. The use of pesticides is not always efficient because even alow density of wireworms can affect the field and be responsible for important economical damage. Thewithdrawal of many insecticide formulations (including neonicotinoids), leaves producers without controlmethods. 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 agentsand 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-killstrategy of wireworm’s control. To do so, several sub-goals have been set : (a) developing behavioral assayson the wireworm orientation behavior (b) profiling volatile organic compounds (VOCs) released in therhizosphere 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. Afterfield trials, we tested the hypothesis according to which different maize varieties have differentialsusceptibility/attraction to wireworms because they emit different VOCs from the roots. The work wasfocused on the VOCs quantification from the roots of two maize varieties having experienced contrastedlevels of wireworm infestation in the field. We showed that the least susceptible variety released a morediverse blend of VOCs, including large amounts of hexanal, heptanal and 2,3-octenanedione. Behavioralassays showed that wireworms were strongly attracted to VOCs released by maize roots. However, whenwireworms 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 ofmaize plants that are already under attack by conspecifics. Seven compounds from plants and larvae havebeen identified, amongst which 2-Pentylfuran was present in high quantity, particularly in treatments wherelarvae 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 againstwireworms. We first compared the efficiency of several EPN strains through classical screening laboratoryassays (contact application of EPNs). Then, we developed an attract-and-kill system proposing alginate beadsfilled with EPNs and spiked with potato extract to larvae. The potato extract was previously tested asattractive for wireworms and easy to produce and to use in the laboratory tests. We found that the ingestion ofthese beads led to higher mortality rates. This method also demonstrated some signs of reduction of thewireworms metabolism.Through this work, we were able to deeper study the wireworms orientation behavior in terms of attractionto susceptible maize varieties. We showed that VOCs are part of the cues for the host plant detection. Stillregarding the behavior, the larvae aggregation with conspecifics on plant roots was highlighted for the firsttime, opening an important research path. Finally, the development of the attract-and kill system combiningVOCs 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 theaggregation, the cues for the host selection and preference that could be combined to other biocontrolmethods. 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 ofthis research could lead to effective and innovative solution in wireworms control.