Contrasts of Three Insecticides Resistance Monitoring Methods for Whitefly

Three resistance monitoring methods were tested to evaluate their relative reliability, discriminating ability, convenience, and practicality for monitoring insecticide resistance in Arizona whiteflies. Adult whiteflies were collected from the field and tested in the laboratory with three methods: leaf disk, sticky trap, and vial. Each method was evaluated against two populations divergent in susceptibility using a mixture of Danitol® + Orthene® and two single chemicals, Thiodan® and Danitol®. The Yuma population was relatively susceptible and the Gila River Basin population highly resistant. Correlations of field efficacy and leaf disk bioassays were conducted with the Yuma population and a comparatively resistant Maricopa population. At each location egg, immature, and adult whitefly densities were monitored before and after Danitol® + Orthene® treatments and resistance estimates were also monitored in the populations using leaf disk bioassays. Our results illustrated that the leaf disk method had the greatest discriminating ability between susceptible and resistant populations. The results also indicated that the vial method was the most practical, and that the sticky trap method was good at discriminating between populations that have large differences in susceptibility. The field efficacy trials indicated results from leaf disk assays reflected what had occurred in the field Introduction Since 1990, whitefly, Bemisia tabaci ( Gennadius), (a.k.a. Bemisia argentifolii Bellows and Perring) has been a very serious threat to production of cotton, vegetables, and melons in Arizona (Byrne et al. 1990). The increased severity of whitefly in the Southwest has been attributed to the establishment and subsequent predominance of a distinctly new form of the pest (Brown et al. 1995). This new biotype (or species) has proven to be much more refractory to insecticides than previous populations. Statewide monitoring of whitefly resistance has been conducted by our laboratory since 1994 in cooperation with the USDA Western Cotton Research Laboratory. These statewide surveys of resistance have confirmed the existence of serious resistance problems in some areas of Arizona (Dennehy et al. 1995). Monitoring insecticide resistance in whitefly with conventional bioassay is costly and difficult but is currently the only effective way to detect and manage resistance in this pest. Worldwide, three very different bioassays have been used for this purpose: the leaf disk method, the sticky trap method, and the vial method. These methods, however,