The silverleaf whitefly Bemisia argentiflolii Bellows and Perring has become one of the most important insect pest in the agricultural areas of northwestern Mexico. After the middle of the 1990’s the problem is no longer the huge populations observed, but the geminivirus transmission. In order to reduce the damage caused by this insect, growers response has been the overuse of insecticides without any strategy for resistance management. This situation creates several problems including soil and fruit contamination, health hazards for field workers, decimation of natural enemies and insecticide resistance. In order to generate data on the level of resistance that whiteflies have developed to various commonly used chemical groups of insecticides, a resistance monitoring program was established in some of the agricultural areas of Mexico. The glass vial bioassay was used as a monitoring tool and technical grade insecticides dissolved in acetone were used to coat the inner surface of the vials. Twenty whiteflies were introduced in each 20 ml vial and mortality and survival were determined 3 hours later. Diagnostic concentrations were decided based on data from bioassays performed on a whitefly population collected from La Paz South Baja, California that had not been exposed to insecticides for two years. Results obtained in 1999 indicated that there was a differential response to insecticides in whitefly populations from northwestern Mexico. The most resistant population to cypermethrin, endosulfan and methyl parathion was collected in Culiacan, Sinaloa from tomato. The population from Ensenada, Baja California also collected from tomato, followed second in resistance to these products and first in resistance to methamidophos. The whitefly population from La Laguna in Coahuila and the population from the Yaqui Valley in Sonora were the most susceptible to all the insecticides evaluated.

The glass vial technique (Cahill and Hacker 1992) was used for resistance monitoring; 20 ml scintillation vials were coated in the inner surface by introducing 1 ml of technical grade insecticide dissolved in acetone. Once the insecticide solution was deposited in the vials, they were rolled for 15 min. in a conventional hot-dog roller to evenly coat the inner surface as the acetone evaporated. Treated vials were then placed in front of an electric fan for 15 min. ventilation to eliminate all acetone residues. Each vial was capped with a plastic cap that had two perforations, one covered with cloth screen to allow ventilation and the other plugged with a piece of paper until used.