ASSESSMENT OF 2 BIOASSAY TECHNIQUES FOR RESISTANCE MONITORING OF SILVERLEAF WHITEFLY (HOMOPTERA, ALEYRODIDAE) IN CALIFORNIA

The relative toxicities of bifenthrin and endosulfan to silverleaf whi teflies, Bemisia argentifolii Bellows & Perring, were compared by 2 me thods-treated glass vials and insecticide-coated yellow sticky cards-i n tests in 4 field populations from cotton in Imperial Valley, Califor nia. Both techniques detected relative susceptibilities of B. argentif olii adults to bifenthrin and endosulfan. The LC(50)s of bifenthrin ra nged from 0.007 to 0.07 mu g/vial for the 4 populations with the vial technique and from 0.77 to 3.55 mu g(AI)/ml with the yellow sticky car d technique. Relative tolerance levels to bifenthrin ranging from 2.2- to 10.1-fold were observed in the 4 populations with the yellow stick y card technique. The vial technique showed lower relative response le vels (0.2- to 2.3-fold). With endosulfan, the range was similar for bo th techniques, (1.2- to 5.1-fold with vials; 0.5- to 7.0-fold with the yellow sticky card technique). The frequency of resistance to 4 insec ticides compared with a greenhouse susceptible strain was evaluated wi th the vial system against 11 populations of silverleaf whiteflies col lected from various crops in Imperial Valley during 1992. Resistance t o endosulfan was substantial (maximum, 48-fold), but relative toleranc e was considerably lower to bifenthrin (RR = 0.4- to 8-fold). The inse cticide-coated yellow sticky card technique was used for monitoring in Imperial Pale Verde, and San Joaquin Valleys in California in the fol lowing year. Considerable differences in resistance ratios for the 4 i nsecticide treatments were observed among whitefly populations from th e 3 geographic regions. Bioassays indicated that methomyl was effectiv e against adults in all locations.