175 - Larvicide Resistance and Cross-resistance in the Culex pipiens assemblage

Abstract: Larvicides are available with a variety of active ingredients and formulations allowing for tailored larvicide solutions based on location-specific needs, including a number of biopesticides with limited impact on non-target organisms. As such, larvicides are used extensively in West Nile virus (WNV) mitigation strategies. Broadly, larvicide use has led to the development of larvicide resistance and cross-resistance to multiple active ingredients. Previous studies documenting cross-resistance in mosquito populations to active ingredients with different modes of action is of particular concern, as optimal mosquito control efforts include rotating pesticide products with different modes of action to prevent and remedy larvicide resistance. This study was designed to assess the spectrum of larvicide resistance phenotypes in WNV vector species in the Culex pipiens assemblage using Cx. quinquefasciatus and Cx. pipiens/quinquefasciatus hybrid colonies selected for resistance to S-methoprene and Lysinibacillus sphaericus, respectively. For each colony, we characterized the toxicity of S-methoprene, L. sphaericus, pyriproxyfen, and spinosad as measured by LC50 and LC90 values, then calculated resistance ratios relative to a susceptible laboratory strain. The S-methoprene-resistant Cx. quinquefasciatus used in this study showed cross-resistance to both pyriproxyfen and L. sphaericus. Likewise, the L. sphaericus-resistant Cx. pipiens/quinquefasciatus hybrid colony exhibited cross-resistance to S-methoprene but were susceptible to pyriproxyfen. Notably, cross-resistance to S-methoprene in L. sphaericus-resistant populations has not been previously documented. Additionally, both resistant colonies were susceptible to spinosad. These findings reveal a reciprocal cross-resistance relationship between S-methoprene and L. sphaericus, which should be taken into account when devising product rotation strategies.