The residual contact toxicity of three benzofuranoids (Z)-butylidenephthalide (1), (3S)-butylphthalide (2), and (Z)-ligustilide (3) identified in the rhizome of Cnidium officinale (Apiaceae) to B- and Q-biotype females of Bemisia tabaci was evaluated using a leaf-dip bioassay. Results were compared with those of eight conventional insecticides. Based on 24 h LC(50) values, (Z)-butylidenephthalide (254 ppm) and (Z)-ligustilide (268 ppm) were more toxic than (3S)-butylphthalide (339 ppm) against B-biotype females, whereas (Z)-ligustilide (254 ppm) and (3S)-butylphthalide (338 ppm) were more toxic than (Z)-butylidenephthalide (586 ppm) against Q-biotype females. Thiamethoxam, imidacloprid, and acetamiprid differ significantly in toxicity between the B- and Q-biotype females (LC(50), 1.7 to 11.6 vs 364.5 to >3000 ppm). This original finding indicates that the phthalides and the neonicotinoids do not share a common mode of action or elicit cross-resistance. Structure-activity relationship indicates that the presence of conjugation rather than aromaticity appeared to play an important role for phthalide toxicities to the B-biotype females. Global efforts to reduce the level of highly toxic synthetic insecticides in the agricultural environment justify further studies on C. officinale rhizome-derived materials as potential insecticides for the control of B. tabaci populations.