Seven F1 strains of Aedes aegypti (L.) were evaluated by bottle bioassay for resistance to the pyrethroids d-phenothrin, permethrin, deltamethrin, λ-cyalothrin, bifenthrin, cypermethrin, á-cypermethrin, and z-cypermethrin. The New Orleans strain was used as a susceptible control. Mortality rates after a 1 h exposure and after a 24 h recovery period were determined. The resistance ratio between the 50% knockdown values (RRKC50) of the F1 and New Orleans strains indicated high levels of knockdown resistance. The RRKC50 with α-cypermethrin varied from 10 to 100 among strains indicating high levels of knockdown resistance. Most of the strains had moderate resistance to (d-phenothrin. Significant but much lower levels of resistance were detected for λ-cyalothrin, permethrin, and cypermethrin. For z-cypermethrin and bifenthrin, only one strain exhibited resistance with RRKC50 values of 10- and 21-fold, respectively. None of the strains showed RRKC50 >10 with deltamethrin, and moderate resistance was seen in three strains, while the rest were susceptible. Mosquitoes from all strains exhibited some recovery from all pyrethroids except d-phenothrin. Regression analysis was used to analyze the relationship between RRKC50 and RRKC50. Both were highly correlated (R2 = 0.84-0.97) so that the slope could be used to determine how much additional pyrethroid was needed to ensure lethality. Slopes ranged from 0.875 for d-phenothrin (RRLC50 RRKC50) to 8.67 for ë-cyalothrin (∼8.5-fold more insecticide needed to kill). Both RRLC50 and RRKC50 values were highly correlated for all pyrethroids except bifenthrin indicating strong cross-resistance. Bifenthrin appears to be an alternative pyrethroid without strong cross-resistance that could be used as an alternative to the current widespread use of permethrin in Mexico.
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Vol. 106 • No. 2