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Bacterial insecticides play an increasingly important role in mosquito control. To establish guidelines for detecting resistance at an early stage, information on natural variation in susceptibility of insect populations to these insecticides is needed. Between 1990 and 1993, the susceptibility of Culex pipiens L. complex to Bacillus thuringiensis subsp. israelensis de Barjac and/or Bacillus sphaericus Neide was determined in 31 collections from California. These collections were undertaken before the widespread use of B. thuringiensis subsp. israelensis and before the registration of B. sphaericus in California. Seven collections from the Mediterranean island of Cyprus, where no microbial insecticides have been used, also were tested. The 1990-1991 California collections exhibited limited variation in susceptibility to B. thuringiensis subsp. israelensis. LC50 and LC95 values spanned about a three-fold and four-fold range, respectively. The 1993 Cyprus collections exhibited both higher mean LC values, and greater variability in those values, than the California collections. The LC50s for the Cyprus collections varied over a 10-fold range, whereas the LC50s varied over a 12.5-fold range. Variation in susceptibility to B. sphaericus among the 1991 California collections was about five-fold at the LC50 and LC95. No significant geographic variation in susceptibility to B. thuringiensis subsp. israelensis was observed among regions within California. Although variation in susceptibility was limited among California collections, the greater variability observed among the Cyprus collections and between the Cyprus and California collections illustrates the importance of establishing regional baselines to monitor accurately for changes in susceptibility.

Amplification of the esterase Bi gene is responsible for insecticide resistance in the mosquito Culex quinquefasciatus. We used a mating scheme to isolate chromosomes carrying amplified esterase genes from a long-selected laboratory strain (Tem-R) to determine whether observed variation in esterase activity had a genetic basis. The amplified esterase genes segregated as a block and a possible newly arisen esterase B 1 copy-number variant was found among the progeny of females which carried amplified B 1 genes on only one homologue. A quantitative genetic analysis found significant genetic variation of esterase activity among families which carried different amplification-bearing chromosomes from the Tem-R strain. Esterase B 1 copy-number variation among these Tem-R chromosomes is the most likely basis for the observed genetic variation in esterase activity.