Insecticide resistance and the efficacy of ground ULV applications in Fresno County, CA

During July _ September 2009, ground ultra low volume (ULV) applications of various registered mosquito adulticide formulations were evaluated in a field assay against both wild populations and an insecticide-susceptible laboratory colony of Culex pipiens sensu lato. Initial applications with piperonyl butoxide (PBO) synergized formulations of natural pyrethrins and selected synthetic pyrethroids indicated that the wild populations in question had varying degrees of resistance to these products. Laboratory bottle-bioassay testing confirmed these observations. Adult female surveillance using carbon dioxide baited CDC traps and Bermuda grass infusion gravid traps were also utilized to quantify population impacts of ground ULV applications. There was no significant difference in pre and post treatment collections made at sites throughout the treatment area. Surveillance collections further supported the assumption of resistance and a lack of measurable control. A combination of malathion (Fyfanon�) and natural pyrethrins, synergized at 10:1 with PBO (Evergreen�), had significant resistance-breaking properties. This study demonstrates the existence of significant pesticide resistance in wild Cx. pipiens s. l. populations in Fresno County, as well as the impact of barriers in urban and rural environments on drift, effective swath and reduced efficacy of ground ULV applications.

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