Residual Activity of Pyriproxyfen Against Mosquitoes in Catch Basins in Northwestern Riverside County, Southern California

A field study was carried out on the year-long residual activity of the insect growth regulator (IGR) pyriproxyfen (Nylar 0.5G) in comparison with methoprene (Altosid® XRP Pellets) against mosquito developmental stages in catch basins in northwestern Riverside County, southern California. Pyriproxyfen was applied at 75, 100, 125, 150, 175 g per catch basin and methoprene at 3.5 g per catch basin. A total of 80 catch basins (10 per each treatment and 20 for control) were used. Posttreatment observations of catch basins were carried out at weekly intervals, with all pupal collections reared to adults. Mosquito species composition in this study, consisting mostly of Culex species (693), was predominated by Cx. quinquefasciatus (92.8%), followed by Cx. erythrothorax (5.5%), Cx. tarsalis (1.2%), Cx. stigmatosoma (0.3%), and Cx. thriambus (0.2%). Activity of both IGRs was expressed as percent inhibition of adult emergence (% IAE). Data generated on % IAE showed that, like methoprene, pyriproxyfen provided complete control of mosquitoes at 75, 125, and 175 g per catch basin up to 50 wk posttreatment at the Riverside amusement park, whereas its activity against mosquitoes in catch basins treated with 100 g and 150 g at the Eastvale site was short-lived, up to 48 wk. Water samples, bioassayed against laboratory-reared, 4th-stage larvae of Cx. quinquefasciatus 1–2 wk after the 50-wk-long study, showed evidence of significant % IAE (∼50) by pyriproxyfen at the 2 higher rates (125 g, 175 g) used at the amusement park. In conclusion, pyriproxyfen can be used to effectively control mosquitoes in catch basins for 48–50 wk, depending on the rate of application.