| The toxicity of imidacloprid, acetamiprid and thiamethoxam to the adults, eggs and nymphs of Bemisia tabaci(Gennadius)was determined by using adult leaf-dip bioassay, egg-dip assay and nymph-dip assay. The values of LC50 of imidacloprid, acetamiprid and thiamethoxam were 53.54mg·L-1, 12.84mg·L-1, 52.9mg·L-1 to the adults, 83.77mg·L-1, 17.98mg·L-1, 210.65mg·L-1 to the eggs, and 44.98mg·L-1, 8.27mg·L-1, 42.08mg·L-1to the second instar nymphs of B. tabaci respectively. The values of LC50 of imidacloprid to the eggs of B. tabaci were 83.77 mg·L-1 by using egg-dip assay, and 51.80 mg·L-1 by using systemic bioassay respectively, which showed higher toxicity of imidacloprid to the eggs by systemic conduction than that by leaf conduction. To evaluate the sublethal effects of imidacloprid, we used LC40 and LC20 concentrations at 30mg·L-1and 10 mg·L-1 to the adults, 40mg·L-1and 12.5mg·L-1 to the eggs, 30mg·L-1and 15 mg·L-1 to the second instar nymphs respectively.The selectivity of B. tabaci were investigated based on free diffusion observations in petri dishes, the results indicated that pretreatment by imidacloprid did not affect the selectivity of B. tabaci. Under the selection condition, the adults of B. tabaci preferenced to the no-treatment cotton leaf, the numbers of B. tabaci landing on the no-treatment cotton leaf were higher than that on the treatment, which indicated that sublethal doses of imidacloprid had certain deterrent effects on B. tabaci. The honeydew excretion of female adults were 3.67mm2 and 2.16mm2 respectively after treated by LC20 and LC40 of imidacloprid, and were significantly lower than that of no-treatment (33.33mm2), which indicated that the sublethal doses of imidacloprid had antifeedant effects on the adults of B. tabaci. Moreover, both sublethal doses of imidacloprid significantly reduced the number of eggs laid by B. tabaci daily. But this reduction was transient, as adults of B. tabaci were transferred to the fresh leaf and detached from intimidation of the sublethal doses of imidacloprid, the number of eggs laid daily could recovered in short time.With dipping method, the egg hatching rate of B. tabaci treated by LC40 and LC20 of imidacloprid were only 76.57% and 54.78% respectively, which was significantly lower than that of no-treatment (92.21%). The egg developmental duration of B. tabaci was 6.34d and 6.29d respectively, which had no significant differences with no-treatment (6.43d). The results indicated that sublethal doses of imidacloprid significantly reduced the egg hatching rate of B. tabaci, but did not affect the growth of survived egg.After treated by LC40 and LC20 of imidacloprid for the 2nd instar nymphs of B. tabaci, the development duration of survived nymphs, and the eclosion rate and longevity of adults had no significant difference compared with no-treatment. However, the number of egg laid by per female from survived nymph treated by LC20 of imidacloprid was 332.23, which was significantly higher than that of no-treatment (182.86).Both sublethal doses (LC20 and LC40) of imidacloprid did not affect adult longevity and fecundity on parent generation. Both sublethal doses (LC20 and LC40) of imidacloprid also did not affect the egg hatching rate, egg development duration, nymphal survival and adult eclosion rate on F1 generation, but the LC40 of imidacloprid significantly affected the sex ratio on F1 generation. |
