Baseline Susceptibility of Bemisia tabaci B Biotype (Hemiptera: Aleyrodidae) Populations from California and Arizona to Spiromesifen

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 & Perring) (Hemiptera: Aleyrodidae) from Califor- nia and Arizona by laboratory bioassays. Newly emerged Þrst instars 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 whitesies 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 Þrst instars 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 resect natural variation in spiromesifen susceptibility among all the test populations, possibly due to previous exposure to insecticides at each location. The effectiveness of spiromesifen also was evaluated against all immature stages of whitesies from three Þeld and two resistant strains. Spiromesifen was signiÞcantly more active against early instars of whitesies based on lower LC50 values recorded compared with the fourth 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 Þndings suggest that spi- romesifen should be considered an ideal candidate for whitesy resistance management programs in rotation with other effective chemistries.