Persistence of Bacillus thuringiensis israelensis (Bti) in the environment induces resistance to multiple Bti toxins in mosquitoes

BACKGROUND: The simultaneous production of six different toxins by Bacillus thuringiensis israelensis (Bti) is thought to delay the evolution of resistance in treated mosquito populations. Recent studies have shown that Bti can persist and proliferate in the environment, thereby imposing continuous selective pressure on mosquito populations, raising concerns about the long-term effectiveness of this bioinsecticide. In order to evaluate the effect of Bti persistence on the evolution of resistance, the authors selected a laboratory Aedes aegypti L. strain with field-collected leaf litter containing Bti toxins. RESULTS: It is shown that resistance to each individual Bti toxin (up to 30-fold) can be obtained after only a few generations of selection. However, the resistance to commercial Bti and to environmental Bti remains low (twofold and 3.4-fold respectively) in the selected strain. Furthermore, some selected individuals exhibited resistance to Cry4B but not to Cry4A, suggesting that two distinct resistance mechanisms are involved in the resistance to these two toxins. CONCLUSION: Considering that resistance to Cry toxins might act as a first step to resistance to a complete Bti toxin mixture, the present results highlight the importance of testing each toxin individually in order accurately to monitor Bti toxin resistance evolution in field populations. (C) 2010 Society of Chemical Industry