Selection for Imidacloprid Resistance in Silverleaf Whiteýies from the Imperial Valley and Development of a Hydroponic Bioassay for Resistance Monitoring

A Ðeld-collected population of the silverleaf whiteÑy, Bemisia argentifolii, was selected with the nicotinyl compound, imidacloprid, over 32 generations to determine if resistance would develop when maintained under continuous selection pressure in a greenhouse. Resistance was slow to increase at Ðrst with low to moderate levels of resistance (RR from 6to 17-fold) in the Ðrst 15 generations of selection. Further selection steadily led to higher levels of resistance, with the greatest resistance ratio at 82-fold, the gradual rise suggesting the involvement of a polygenic system. At the end of the selection, slopes of probit regressions were substantially steeper than earlier, indicating 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 imidacloprid in the selected whiteÑy strain and in seven Ðeld-collected strains from Imperial Valley, California. Six out of seven Ðeld-collected strains exhibited low values 50 (0É002 to 0É512 mg ml~1) compared to the selected resistant strain, with one exception where the was 0É926 mg ml~1 (RR\ 15É0). Variation in 50 responses to imidacloprid in the Ðeld strains suggest that this technique is sufficiently sensitive to detect di†erences in susceptibilities of whiteÑy populations. The imidacloprid-resistant strain showed no cross-resistance to endosulfan, chlorpyrifos or methomyl (RR ranging from 0É4to 1É5-fold). A low level of cross-resistance was observed to bifenthrin in the IM-R strain at 7-fold. The success of selection for resistance to imidacloprid has serious implications for whiteÑy control programs that rely heavily on imidacloprid. Pestic. Sci., 51, 419È428, 1997 No. of Figures : 1. No. of Tables : 4. No. of Refs : 27