Effect of imidacloprid on the silverleaf whitefly, Bemisia argentifolii Bellows and Perring (Homoptera: Aleyrodidae), and whitefly parasitism

Bioassays were conducted under greenhouse conditions to determine the effect of imidacloprid on adult and nymphal stages of the silverleaf whitefly, Bemisia argentifolii Bellows and Perring, and parasitism by Encarsia fromosa (Gahan). A flowable formulation (240 g litre(-1)) of imidacloprid at six rates (0.09, 0.04 and 0.02 g a.i. litre(-1) pot volume for experiments 1 and 2; 0.009, 0.004, and 0.002 g a.i. litre(-1) pot volume for experiment 3) was evaluated. After a 48 h exposure to treated plants, high mortality of adult whitefly (> 94%) was observed. Adults exposed to poinsettias treated 150 days earlier also had significantly greater mortality (>79%) than the adults on control plants. When exposed to treated plants for only 6 h, >65% of adults were killed. All three rates of imidacloprid caused >97% mortality of immature whiteflies by day 19. When treated plants were continually exposed to adult whiteflies, immature mortality was 100% for the three higher rates of imidacloprid up to and including 88 days after treatment. During the same time, emerging adults were reduced significantly. Immatures reinfested on plants treated 161 days earlier, incurred 80% mortality at the higher rate 0.09 g a.i. litre-pot volume and 38% mortality at 0.02 g a.i. litre(-1) pot volume. At lower treatment rates, results varied. At 0.009 g a.i. litre(-1) of pot volume, mean percentage whitefly mortality (65%) 25 days after treatment was significantly higher than the controls; however, whitefly mortality at 0.004 and 0.002 g a.i. litre(-1) pot volume was not significantly different from controls or plants treated with the higher rate. Parasitoids could develop to the adult stage on whiteflies infesting imidacloprid treated plants. Parasitism occurred at low levels (<10%), due to high levels of whitefly mortality on treated plants. No phytotoxicity was observed for any treatment throughout the length of the trials that lasted through flowering.