Extreme resistance to S-methoprene in field-collected Culex pipiens (Diptera: Culicidae) across the Chicago, IL region

Abstract

Insect growth regulators, like S-methoprene, are heavily relied upon worldwide for larval mosquito chemical control due to their target specificity and long-lasting effects. In this study, susceptibility to S-methoprene was evaluated in Culex pipiens, a globally important vector species. Populations from 14 sites throughout the Chicago area with a long history of S-methoprene use and two sites with minimal use in Wisconsin were examined. Using a bioassay methodology and probit analyses, LC₅₀ and LC₉₀ values were calculated and compared to a susceptible laboratory strain to develop resistance ratios, then categorized for resistance intensity. The resistance ratios observed required the addition of another category, termed 'extreme' resistance, indicating resistance ratios greater than 100. 'Low' to 'extreme' levels of resistance to S-methoprene were detected throughout Illinois populations, with resistance ratios ranging from 2.33 to 1010.52. Resistance was not detected in populations where S-methoprene pressure has been very limited. These 'extreme' resistance ratios observed have never been documented in a wild vector species mosquito population. The relationships between historical S-methoprene use, resistance detected with laboratory bioassays, and the potential for field product failure remain unclear. However, the profound resistance detected here demonstrates a potential critical threat to protecting public health from mosquito-borne diseases.

Keywords: Insect growth regulator; Larvicide; Mosquito control; Susceptibility; Vector.

Figure 1

Map of study area, triangles indicate sampling sites. (A) Map of Wisconsin and Illinois, (B) The five MCOs in Cook County, (C) Sampling sites within each MCO.

Figure 2

Resistance ratios of field-collected C. pipiens compared to a lab susceptible strain (COL), calculated using LC₅₀ values (A) and LC₉₀ values (B). ‘High’ resistance indicated by dashed line (resistance ratio = 10) and ‘extreme’ resistance indicated by solid line (resistance ratio = 100).