Indoxacarb, a sodium channel-blocking insecticide, has been in widespread use for German cockroach control in the United States since 2006. A two-tiered indoxacarb susceptibility monitoring strategy was previously developed as a first step toward determining indoxacarb susceptibility levels in German cockroach field populations. This strategy entails: (tier 1) testing field-collected populations in vial bioassays at two diagnostic concentrations; and (tier 2) testing populations at three diagnostic doses in oral (feeding) bioassays with treated bait matrix. In the current study the two-tiered technique was implemented to evaluate field (n = 14) and susceptible laboratory (n = 2) strains collected from 13 different U.S. locations. Our hypothesis was that at least some of the field-collected populations would display significant survivorship in both bioassays relative to susceptible laboratory populations. In agreement with this hypothesis, significantly reduced susceptibility was detected in 13 and 7 field strains with vial and feeding bioassays, respectively. In general, the lower number of strains displaying reduced susceptibility in feeding bioassays (seven strains) supports previous findings that indoxacarb is more toxic via ingestion. Although these findings suggest a reduced risk for resistance selection via feeding on indoxacarb-containing baits, they also suggest a need for proactive resistance management with respect to both spray and bait products.
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