Resistance to S-Methoprene Correlates with Pyriproxyfen Resistance in Field-Collected Culex pipiens

Area of Science:

• Medical Entomology
• Insecticide Resistance Studies
• Vector-Borne Disease Control

Background:

• Insecticide resistance compromises Integrated Mosquito Management (IMM) programs.
• Cross-resistance between insecticides complicates resistance management strategies.
• Previous studies suggested potential cross-resistance between S-methoprene and pyriproxyfen in laboratory mosquitoes.

Purpose of the Study:

• To investigate field-evolved cross-resistance to pyriproxyfen in S-methoprene-resistant *Culex pipiens* populations.
• To assess the impact of S-methoprene exposure and pyriproxyfen use history on resistance levels.
• To determine the prevalence of pyriproxyfen resistance in mosquito populations in the Chicago metropolitan area.

Main Methods:

• Dose-response bioassays were conducted on 31 field-collected *Culex pipiens* populations.
• Populations exhibited varying levels of prior exposure to S-methoprene and pyriproxyfen.
• Resistance ratios (RR50) were calculated to quantify resistance levels to both insecticides.

Main Results:

• All 31 populations showed high resistance to S-methoprene (RR50 > 10).
• 84% of populations exhibited moderate to high resistance to pyriproxyfen (RR50 > 5).
• Reduced pyriproxyfen susceptibility was observed even in populations with no pyriproxyfen exposure, indicating S-methoprene-mediated cross-resistance.

Conclusions:

• Widespread, high-level pyriproxyfen resistance is reported in *Culex pipiens*, a medically significant mosquito species.
• S-methoprene resistance and pyriproxyfen exposure levels significantly correlate with pyriproxyfen resistance.
• Urgent need for routine resistance surveillance and integrated resistance management using diverse larvicides is highlighted.