Resistance to s-methoprene and Lysinibacillus sphaericus in Culex pipiens from the Chicago area

Insect growth regulators and bacterial midgut disruptors are heavily relied upon for larval mosquito control due to their target specificity and long-lasting effects. Despite decades of use in the Chicago area to control West Nile virus vectors in catch basins, resistance to these active ingredients has not been thoroughly evaluated. In 2023, we evaluated resistance to S-methoprene in 15 Culex pipiens populations throughout the Chicago area. In general, resistance to S-methoprene was not a concern, as juvenile hormone is integral to the mosquito life cycle, and mechanisms behind resistance have not been reported. However, we detected very high, wide-spread resistance to this larvicide. Concerningly, cross resistance to Spinosad and Lysinibacillus sphaericus has been recorded in the literature, so evaluating resistance to these other active ingredients was necessary. In 2024, we evaluated cross resistance between S-methoprene and L. sphaericus in 8 Cx. pipiens populations from the suburbs of Chicago in laboratory, semi-field, and field experiments. Investigations are ongoing, though preliminary results show continued resistance to S-methoprene and resistance to L. sphaericus in the laboratory, as well as sub-optimal control with formulated products (Altosid WSP and VectoLex WSP) in semi-field and field experiments. Resistance to these active ingredients reduces the strategies and chemistries that can be used to control Cx. pipiens, both in Chicago and worldwide. Larvicide resistance needs to be evaluated where these products are being used; mosquito control personnel can no longer assume all larvicides are effective.