From wood protection to health protection: larvicidal potential of formulations containing Sextonia rubra wood residues extract against Aedes aegypti mosquitoes

International audience

anglais. French Guiana is a French overseas territory almost entirely covered by Amazonian rainforest and characterized by its incredible biodiversity. Several woody species harboured by this tropical forest are exploited sustainably for timber industry. They are commercialized notably for carpentry and outdoor applications because of their remarkable natural durability. Among them, stand Dicorynia guianensis Amshoff (Angélique), Qualea rosea Aubl. (Gonfolo rose), Ruizterania albiflora (Warm.) Marc.-Berti (Gonfolo gris) and Sextonia rubra Mez. Van der Werff (Grignon franc), representing around 80% of the global exploitation volume.This industry however generates large volumes of wastes, in particular because more than 50% of the ligneous material are lost between cutting down and sawing. Moreover, the strong demographic growth (around 2.5% per year) in French Guiana will lead to a notable increase of the need for timber, in construction as well as for energy, and the amount of induced wood wastes will increase concurrently. Sawdust residues are used currently only for biomass energy factories, but owing to the presence of molecules displaying interesting biological properties, wood residues and sawdust definitively deserve more attention. In recent years, a rising interest in value-added forest products, and in particular wood extractives, has indeed been observed, with the aim of reducing the lost generated by the forest industry due to undervalorized biomass components (Royer et al., 2013a; Khan et al., 2014; Borges et al., 2019). It is therefore relevant to integrate a new step in forest resources valorisation, putting the bio-refinery concept into practice (Royer et al., 2013b). In fact, regarding their longevity, trees have developed a wide range of chemical defense mechanisms in several organs (leaves, roots, bark, stem) in response to biotic and abiotic stress. The study of these particular traits alongside with morphological and physiological traits and life history parameters is crucial to understand the mechanisms that support numerous ecological processes (Chave et al., 2009). Wood notably provides the tree mechanical strength as well as storage and conduction capacity, alongside with resistance to decay and pathogens or insects attacks. Compounds produced during heartwood formation, a complex process characterized by the programmed death of parenchyma cells in the transition zone and accompanied by high enzymatic activities generating many specialized metabolites from carbohydrate sources stored in the sapwood, are indeed widely acknowledged as one of the most important passive defense mechanisms of the tree. As they are therefore fundamental for species ecological success, but also present the greatest interest for manhood, wood specialized metabolites extensive study should be clearly promoted. They could in particular inspire the search for novel botanical insecticides (Miresmailli and Isman, 2014). As French Guiana faces severe issues linked to insects, notably the strong incidence of vector-borne infectious diseases such as dengue, zika virus or chikungunya, and as vector control remains an important, if not the sole tool to fight diseases spread, we decided to intensify our research on wood extractives and their ability to control the mosquito Aedes aegypti. In previous studies, we identified Sextonia rubra (Mez) Van der Werff (Lauraceae) as a species of particular interest (Falkowski et al. 2016; 2020). This tropical tree is a rather frequent species in the Guyanas, Venezuela and Amazonian Brazil from the Atlantic coast to Manaus and the featured long lasting heartwood drives the wide exploitation of this species in French Guiana forest industry. S. rubra heartwood is indeed known to possess strongly defensive compounds, the two γ-lactones rubrenolide and rubrynolide, exhibiting activities against lignivorous fungi and termites and representing around 50% of heartwood extractives (Rodrigues et al., 2011; Houël et al., 2017). Interestingly, S. rubra wood extract and molecules were also strongly insecticidal when tested against mosquito larvae of the vector Ae. aegypti. Considering this strong bioactivity, some members of our group (WO2016046489A1) filed a patent and a new research project entitled “SextoniAedes” and funded by the European Regional Development (ERDF) is currently ongoing to build upon this discovery.In this project, we aim at gaining inspiration from wood protection on a wide meaning to develop innovative solutions to protect human health. On the one side, we will investigate S. rubra wood extractives, naturally protecting the tree tissue against insects and pathogens attacks, to develop novel botanical insecticides to fight against the mosquito Ae. aegypti, responsible for the spread of numerous pathogens. On the other side, we will propose aqueous-based formulations of S. rubra extract, influenced by the wood protection industry and more particularly the advances observed in green chemistry. We will test these formulations in semi-operational conditions to evaluate their larvicidal activity and remanence under tropical conditions. Moreover, we will concurrently evaluate eco- and cytotoxity of the extract and formulation in accordance to current regulations. Indeed heartwood metabolites, acting as defensive compounds, can be highly toxic molecules. Therefore, its safety for potential users and selectivity against non-targeted organisms has to be carefully tested.In this context, the “SextoniAedes” project aims to study S. rubra defensive heartwood molecules to inspire economically valuable and environmentally friendly solutions for vector control, and set up the valorization of forest extractives from durable wood species in a circular economy approach. This valorization would constitute a link between the wood industry and the biomass energy sector, with the aim of reducing the environmental impact of wastage products from forest industry. At the same time, this project wants to promote opportunities related to green technology or green innovation. Through concepts such as bio-inspiration or bio-refinery, durable trees give us the potential to inspire scientific innovation and stimulate economic sectors, in a sustainable development context, supporting the ambition to strive for a development model that is fair, and respectful of people and natural resources.