INSECTICIDE RESISTANCE AND RESISTANCE MANAGEMENT Inter-Regional Differences in Baseline Toxicity of Bemisia argentifolii (Homoptera: Aleyrodidae) to the Two Insect Growth Regulators, Buprofezin and Pyriproxyfen

A survey of 53 Bemisia argentifolii Bellows & Perring populations from different agricultural regions in California and Arizona was conducted from 1997 to 1999 to establish baseline toxicological responses to buprofezin and pyriproxyfen. Although both compounds proved to be highly toxic even in minute quantities to speciÞc stages, geographical and temporal differences in responses were detected using a leaf spray bioassay technique. Monitoring for three years revealed that six to seven populations had higher LC50 values but not greater survival when exposed to these two insecticides. A signiÞcant difference in relative susceptibility to buprofezin was Þrst observed in late season1997 inSanJoaquinValleypopulationswithLC50s ranging from16 to22mg(AI)/liter 1 comparedwith LC50s of 1 to 3mg (AI)/liter 1 in Imperial, Palo VerdeValley and Yuma populations. Whiteßies collected in subsequent years from these and other locations showed an increase in susceptibility to buprofezin. Regional differences in susceptibilities to pyriproxyfen were minimal within the same years. Three years of sampling revealed consistently higher LC50s to pyriproxyfen in populations from Palo Verde Valley, CA, compared with whiteßies from Imperial, San Joaquin Valley or Yuma. As was the case with buprofezin, a decline in LC50s to pyriproxyfen was observed in whiteßies from all locations sampled in 1999. However, no correlation was observed between buprofezin and pyriproxyfen toxicity in any of the strains. The variable toxicities observed to both compounds over a period of 3 yrmay be due principally to inherent differences among geographical populations or due to past chemical use which may confer positive or negative cross-resistance to buprofezin or pyriproxyfen.