Resistance to Bacillus sphaericus in Culex pipiens (Diptera : Culicidae) interaction between recessive mutants and evolution in southern France

In southern France, failure to control Culex pipiens L. with Bacillus sphae ricus Neide toxin (Bs) was first detected in 1994, at the extreme east of t he Languedoc-Roussillon coast. This failure was due to a single recessive m utant, sp-1(R). Two complementary strategies were used to test whether sp-1 (R) had invaded the Bs-controlled area by 1998. First, a strain (BP) was se lected from resistant larvae sampled in the western part of the Bs-controll ed area. In BP strain, resistance involved a single recessive gene, sp-2(R) , distinct from sp-1(R), that conferred a similarly high resistance in the homozygous state (approximate to6,000-fold). Combining one copy of sp-1(R) and one of sp-2(R) conferred a > 100-fold resistance. Second, Bs-resistance was monitored among the offspring of field females crossed to sp-1(RR) hom ozygous males. Females were sampled in 20 localities of southern France and three localities of the Llobregat delta (Barcelona, Spain) where C. pipien s control is also intensive. The 537 females in the study produced enough l arvae to infer their genotype: 462 progenies were susceptible and the survi val rate of 51 others was explained by the presence of sp-1(R) and/or sp-2( R). The remaining 24 cases indicated that other factors could confer resist ance when combined with sp-1(R). The current data showed that, even when re cessive, resistant mutants can rapidly increase in frequency, providing som e interactions that protect them from disappearance. We discuss the consequ ences of this finding on the current strategies aimed to avoid or delay res istance in the pests controlled with B. sphaericus or B. thuringiensis Berl iner toxins.