Biological fitness of Culex quinquefasciatus (Diptera: Culicidae) susceptible and resistant to Bacillus sphaericus

Biological fitness of laboratory and field-collected strains of southern house mosquito, Culex quinquefasciatus Say, susceptible and resistant (37- and 31-fold upon selection) to the microbial agent, Bacillus sphaericus, were compared in the absence of B. sphaericus. The resistant strains showed significantly lower fecundity and fertility but they had significantly higher survival rates than the susceptible strains. The preadult stages from females of resistant strains developed at slightly faster rates than those of the susceptible strains, which could result in a shorter generation time. However, lower fecundity was likely to lead to overall lower population growth rates than in the susceptible strains. Data provided evidence that the resistant strains exhibited fitness disadvantages in the absence of B. sphaericus. We suggest that once resistance to B. sphaericus is detected in the field, its use should be discontinued until the mosquito population becomes susceptible again because of the decline in number of resistant individuals. A strategy of resistance management by rotation of insecticides is discussed.