Bacillus thuringiensis
subsp.
israelensis (
Bti) is increasingly used worldwide for mosquito control and is the only larvicide used in the French Rhône-Alpes region since decades. The artificial selection of mosquitoes with field-persistent
Bti collected in breeding sites from this region led to a moderate level of resistance to
Bti , but to relatively high levels of resistance to individual
Bti Cry toxins. Based on this observation, we developed a bioassay procedure using each
Bti Cry toxin separately to detect cryptic
Bti -resistance evolving in field mosquito populations. Although no resistance to
Bti was detected in none of the three mosquito species tested (
Aedes rusticus
,
Aedes sticticus
and
Aedes vexans
), an increased tolerance to Cry4Aa (3.5-fold) and Cry11Aa toxins (8-fold) was found in one
Ae. sticticus population compared to other populations of the same species, suggesting that resistance to
Bti may be arising in this population. This study confirms previous works showing a lack of
Bti resistance in field mosquito populations treated for decades with this bioinsecticide. It also provides a first panorama of their susceptibility status to individual
Bti Cry toxins. In combination with bioassays with
Bti , bioassays with separate Cry toxins allow a more sensitive monitoring of
Bti -resistance in the field.
