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

## TL;DR

Mosquitoes in Chicago show high resistance to S-methoprene and also to pyriproxyfen, a related larvicide, which could undermine mosquito control efforts.

## Contribution

First report of widespread high-level pyriproxyfen resistance in a medically significant mosquito species.

## Key findings

- All 31 Culex pipiens populations showed high resistance to S-methoprene.
- 84% of populations were at least moderately resistant to pyriproxyfen, even in areas where it was never used.
- Resistance levels to S-methoprene and pyriproxyfen were significantly correlated.

## Abstract

The widespread problem of insecticide resistance is making it harder to protect communities from mosquito-borne illnesses like West Nile virus (WNV), forcing mosquito control teams to constantly look for new solutions. This study focused on Culex pipiens mosquitoes, the main vector for WNV in the Chicago area, which have been subjected to the larvicide S-methoprene for many years. We found that all 31 mosquito populations from the Chicago area were highly resistant to S-methoprene. We reasoned that this widespread reduced susceptibility to S-methoprene could predispose mosquitoes to a related larvicide called pyriproxyfen, even in areas where pyriproxyfen had never been used. This phenomenon, known as cross-resistance, means that the strategies currently used to reduce resistance, like rotation between or combining these two products, may not be effective. Applying pyriproxyfen only makes the resistance problem worse, though operational outcomes are unknown. This is the first report of such high levels of resistance to pyriproxyfen being found in mosquitoes. Our findings are an urgent warning to mosquito control programs globally. It is important to monitor resistance more closely and adopt new methods that use different types of chemicals to control mosquito larvae.

The increasing prevalence of insecticide resistance threatens the efficacy of Integrated Mosquito Management (IMM) programs, particularly in regions reliant on chemical control for vector-borne disease prevention. Cross-resistance between active ingredients severely complicates essential resistance management strategies like product rotation. The previous literature suggests that laboratory-induced S-methoprene-resistant Culex species may be somewhat cross-resistant to pyriproxyfen, another juvenile hormone analog. This is a critical concern in the Chicago, IL, USA metropolitan area, where pyriproxyfen is used against mosquitoes with reduced susceptibility to S-methoprene. To determine if S-methoprene-resistant Culex pipiens are cross-resistant to pyriproxyfen in nature, we assessed 31 field-collected populations with significant S-methoprene exposure but varying histories of pyriproxyfen use by dose–response bioassays. Culex pipiens from all 31 sites exhibited high resistance to S-methoprene (RR50 > 10), and 84% were at least moderately resistant to pyriproxyfen (RR50 > 5). Reduced susceptibility to pyriproxyfen was confirmed in pyriproxyfen-unexposed populations, demonstrating potential S-methoprene-mediated cross-resistance. The level of S-methoprene resistance and the level of pyriproxyfen exposure significantly correlated with the level of pyriproxyfen resistance. We report the first widespread, high-level pyriproxyfen resistance in any medically significant mosquito species, underscoring the critical need for routine resistance surveillance and the adoption of integrated resistance management tactics utilizing larvicides with distinct modes of action.