A survey of silverleaf whitefly populations from different cotton growing regions in California was conducted in 1997 to establish baseline toxilogical responses to Applaud® and Knack®. Both compounds proved to be highly toxic even in minute quantities. Geographical and temporal variation in responses to each product was observed, but generally was within the range of normal fluctuation. One possible exception was the consistently higher LC50s observed in San Joaquin Valley whiteflies compared to either Palo Verde Valley or Imperial Valley whiteflies. Future surveys will help to establish whether variation in responses to Applaud® and Knack® is due principally to inherent differences among geographical populations or to agro-environmental conditions.