Baseline susceptibility to bacterial insecticides in populations of Culex pipiens complex (Diptera : Culicidae) from California and from the Mediterranean island of Cyprus

Bacterial insecticides play an increasingly important role in mosquito cont rol, To establish guidelines for detecting resistance at an early stage, in formation on natural variation in susceptibility of insect populations to t hese insecticides is needed. Between 1990 and 1993, the susceptibility of C ulex pipiens L. complex to Bacillus thuringiensis subsp, israelensis de Bar jac 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. sphae ricus in California. Seven collections from the Mediterranean island of Cyp rus, where no microbial insecticides have been used, also were tested. The 1990-1991 California collections exhibited limited variation in susceptibil ity to B. thuringiensis subsp, israelensis. LC50 and LC95 values spanned ab out a three-fold and four-fold range, respectively, The 1993 Cyprus collect ions exhibited both higher mean LC values, and greater variability in those values, than the California collections. The LC(50)s for the Cyprus collec tions varied over a 10-fold range, whereas the LC(95)s varied over a 12.5-f old range. Variation in susceptibility to B. sphaericus among the 1991 Cali fornia collections was about five-fold at the LC50 and LC95. No significant geographic variation in susceptibility to B. thuringiensis subsp. israelen sis was observed among regions within California. Although variation in sus ceptibility was limited among California collections, the greater variabili ty observed among the Cyprus collections and between the Cyprus and Califor nia collections illustrates the importance of establishing regional baselin es to monitor accurately for changes in susceptibility.