176 - Investigating resistance to S-methoprene, pyriproxyfen, and spinosad in Culex pipiens from the Chicago region

Abstract: Previously, high levels of resistance to both S-methoprene and Lysinibacillus sphaericus has been detected in Culex pipiens from the Chicago, IL region. Unfortunately, resistance to one active ingredient rarely means the population is only resistant to that chemical – cross resistance is often at play. Previous literature suggests that S-methoprene resistance can cause low levels of cross resistance to spinosad. Cross resistance among the insect growth regulators has not been sufficiently studied. To understand (1) if our local mosquitoes remain resistant to S-methoprene and (2) if this S-methoprene resistance causes cross resistance to spinosad or pyriproxyfen, we evaluated resistance to all three active ingredients in 31 Cx. pipiens populations from the Chicago, IL region. We detected sustained high and extreme levels of S-methoprene resistance in all populations studied. Pyriproxyfen resistance was detected in 26 of 31 populations, even in areas where pyriproxyfen has never been used for mosquito control purposes. Similarly, we detected resistance to spinosad in 11 populations, including in areas without prior spinosad exposure. Importantly, resistance to spinosad has not been documented in the literature, though there are some non-peer reviewed reports. This study confirms that high levels of S-methoprene resistance can cause cross resistance to both pyriproxyfen and spinosad. We hypothesize that because our ‘Culex pipiens’ are hybrids of Cx. pipiens/Cx. pipiens molestus/Cx. quinquefasciatus, in addition to living in highly urban environments, that these populations have intensive non-specific metabolic resistance, resulting in some level of resistance to all insecticides tested to date.