Bti is a bioinsecticide used worldwide for mosquito control. Bti toxicity is due to a toxins-containing crystal, produced by the bacteria Bacillus thuringiensis var. israelensis. Bti is known to have a low persistence in the environment and no firm resistance has been found yet in the field, therefore representing a “safe” alternative to chemical insecticides. Nevertheless, leaf litters collected several months after a treatment exhibited high toxicity against mosquito larvae, due to persistent Bti. The selection of a laboratory mosquito strain of Aedes aegypti with this litter (LiTOX strain) led to a moderate 3.5-fold resistance to Bti but to higher resistance (until 60-fold) to Bti toxins tested separately. This thesis is structured in three main axes. 1. A test has been developed and patented to follow the fate of Bti in the environment. We showed that Bti has recycled in the collected toxic leaf litters and that the different Bti toxins persist differently. 2. The Bti-resistance of the LiTOX strain has been studied by global and functional approaches, leading to a better understanding of the multigenic mechanisms implicated in the resistance phenotype. 3. Cryptic Bti-resistance has been investigated performing bioassays with separate Bti Cry toxins, showing that some populations exhibit increased Cry tolerance. This thesis gives the knowledge and new tools necessary to better understand how Bti can persist and recycle in the environment and how mosquitoes can develop resistance.
