Technical Abstract: : Susceptibility to spiromesifen, a tetronic acid derivative, was determined for three imidacloprid-resistant strains and 12 geographically discrete natural populations of Bemisia tabaci (Gennadius) (= Bemisia argentifolii Bellows and Perring) from California and Arizona by laboratory bioassays. Newly emerged 1st instar whitefly nymphs were sprayed with aqueous serial dilutions of spiromesifen and evaluated for toxicity to establish baseline susceptibility data. Interpopulation variability in susceptibility to spiromesifen was observed among the natural populations of whiteflies up to 29-fold, however, there was only 30- fold difference in susceptibility among natural and resistant populations tested. In general, spiromesifen was quite toxic to 1st instar nymphs across most of their geographic range with LC50 values ranging from 0.210 to 6.08 'g[AI]/ml. The magnitude of variation was smaller among the three-resistant strains. These results suggest that the observed variability reflect natural variation in spiromesifen susceptiblity among all the test populations, possibly due to previous exposure to insecticides at each location. The effectiveness of spiromesifen was also evaluated against all immature stages of whiteflies from three field and two resistant strains. Spiromesifen was significantly more active against early instars of whiteflies based on lower LC50 values recorded compared to the 4th instars. Spiromesifen was effective against the resistant strains including a Q-biotype of B. tabaci from Spain which is highly resistant to neonicotinoids. Results of this study indicate absence of cross-resistance between spiromesifen and more commonly used neonicotinoids. Our findings suggest that spiromesifen should be considered an ideal candidate for whitefly resistance management programs in rotation with other effective chemistries.