SELECTION FOR IMIDACLOPRID RESISTANCE IN SILVERLEAF WHITEFLIES FROM THE IMPERIAL-VALLEY AND DEVELOPMENT OF A HYDROPONIC BIOASSAY FOR RESISTANCE MONITORING

A field-collected population of the silver leaf whitefly, Bemisia arge nti-folii, was selected with the nicotinyl compound, imidacloprid, ove r 32 generations to determine if resistance would develop when maintai ned under continuous selection pressure in a greenhouse. Resistance wa s slow to increase at first with low to moderate levels of resistance (RR from 6- to 17-fold) in the first 15 generations of selection. Furt her selection steadily led to higher levels of resistance, with the gr eatest resistance ratio at 82-fold, the gradual rise suggesting the in volvement of a polygenic system. At the end of the selection, slopes o f probit regressions were substantially steeper than earlier, indicati ng increased homogeneity of imidacloprid resistance in this strain. A hydroponic bioassay featuring systemic uptake of imidacloprid through roots was developed to monitor the changes in resistance to imidaclopr id in the selected whitefly strain and in seven field-collected strain s from Imperial Valley, California. Six out of seven field-collected s trains exhibited low LC50 values (0.002 to 0.512 mg ml(-1)) compared t o the selected resistant strain, with one exception where the LC50 was 0.926 mg ml(-1) (RR=15.0). Variation in responses to imidacloprid in the field strains suggest that this technique is sufficiently sensitiv e to detect differences in susceptibilities of whitefly populations. T he imidacloprid-resistant strain showed no cross-resistance to endosul fan, chlorpyrifos or methomyl (RR ranging from 0.4- to 15-fold). A low level of cross-resistance was observed to bifenthrin in the IM-R stra in at 7-fold. The success of selection for resistance to imidacloprid has serious implications for whitefly control programs that rely heavi ly on imidacloprid.