Toxicity of Biorational Insecticides to Bemisia argentifolii (Homoptera: Aleyrodidae) on Tomato Leaves

Bioassays were conducted to test the toxicity of insecticide leaf residue to adults, and contact toxicity to eggs and nymphs of silverleaf whitefly, Bemisia argentifolii Bellows & Perring. Four insecticides were tested : insecticidal soap (Sunspray oil), mineral oil (MPede), extract of Nicotiana gossei Domin (a detergent-like acylsugar), and bifenthrin (a pyrethroid). Purified tap water was used as a control. Bioassays of adults were conducted by dipping whitefly-free tomato leaves into serial dilutions of the insecticides, air-drying for prescribed periods, and exposing adults to leaves in large cup cages for 24 h. Residues of Sunspray oil caused greatest mortality to adults for up to 5 d after treatment, and the LC 50 of 24 h residue to adults was 0.029%. Two-hour leaf residues of bifenthrin at the field rate (0.06 g [AI]/liter) or higher (0.12-0.24 g [AI]/liter) gave >68% mortality of adults, but efficacy was reduced with residues of 24 h (LC 50 =0.034 g [AI]/liter) or older. Dried residues of insecticidal soap and N. gossei extract were not effective on adults. Contact bioassays were also conducted on tomato leaves infested with uniform cohorts of eggs or nymphs. Response patterns to insecticides were similar among developmental stages of the whitefly, with young nymphs being the most susceptible, followed by older nymphs and eggs. LC 50 s of Sunspray oil to young and old nymphs were 0.032 and 0.088%, and of bifenthrin were 0.001 and 0.106 g (AI)/liter, respectively. Insecticidal soap and N. gossei extract were all effective on young nymphs, even at very low rates (LC 50 , 0.15% and 0.08 g [AI]/liter, respectively), but had no significant effect on eggs. N. gossei extract was effective on older nymphs at low rates (LC 50 = 0.14 g [AI]/liter), whereas insecticidal soap was not (LC 50 =0.51%).