Bioactivities Of Azadirachtin On Laboratory Population Of B-biotype Bemisia Tabaci (Gennadius)

In the recent years, the sweet potato whitefly (Bemisia tabaci Gennadius) causedserious economical damage on cotton, ornamental plants and vegetable crops in China.Heavy application of conventional insecticides (organophosphates, pyrethroids,cyclodienes and neonicotinoids, etc.) coupled with the inherent disadvantages of thosemedicines gives birth to a series of serious problems, such as enviroment pollution,toxicity to human being and animals, killing natural enemy, "3 R" problems (Residue,Resistant and Resurgence). It is suggested that, in the 21st century, the pesticideshould be able to inhibit and regualte growth, development and reproduction of pestsand at the meantime, safeguard human health and reasonable ecosystem balance so asto protect beneficial organisms, control harmful organisms. Azadirachtin (AZ)insecticide is rated to be one of the above-defined pesticides.A tetranortriterpenoid extracted from neem (Azadirachta indicai A. Juss), AZ hasbeen proved to have antifeedant, toxic, growth-regulating, deterrent andanti-oviposition, fecundity-reducing properties on nearly 400 species. To determinethe bioactivities of AZ on the sweet potato whitefly, we investigated the toxicity of 0.3% AZ EC to various stages of the whitefly, bio-activities of sublethal concentrationsof AZ on laboratory population, and deterrene effect of AZ on the adult host-selectionand oviposition, with the aim to guide control of whitefly with AZ.In determining toxicity of AZ on whiteflies, we diluted AZ EC with tap water intoserial concentrations of 0.3125, 0.625, 1.25, 2.5, 5.0 and 10.0 mg/L, with tap wateronly as control. We tested mortality and LC₅₀ of 0.3% AZ EC against whitefly byboth cucumber leaf disc dipping method and petiole wrapping method. Mortalities ofwhiteflies treated by AZ disc dipping method were higher than that treated by petiolewrapping method at the tested AZ concentrations between 0.3125 mg/L and 10.0mg/L, and that mortalities of whiteflies in both the tow bioassay methods weresignificantly dosage-dependent. The medium lethal concentration (LC₅₀) of AZ bydisc dipping method to adults, 1 d and 5 d old eggs and, 1st, 2nd, 3rd and 4th instarnymphs were 3.0246, 2.2019, 5.7109, 0.5909, 0.7465, 1.6948 and 2.6603 mg/L,respectively; and the LC₅₀ of AZ treated by petiole wrapping method were 6.016,4.0321, 6.0537, 0.8614, 0.9941, 2.2419 and 4.4533 mg/L, respectively.On the basis of the above results, bioactivities of AZ at sublethal dosages onBemisia tabaci was studied in laboratory by spraying tap water only (CK) and AZconcentrations of 0.15 mg/L and 0.30 mg/L to 1 d and 5 d old eggs. The results showed that the sublethal AZ as a foliar application significantly delayed immature(from egg to adult emergence) development by 0.92 d to 2.84 d, reduced immaturesurvival rate by 56.3% to 89.6%, retarded nymph size increase, decreased longevity ofadult females by 29.8% to 48.6%, and lowered fecundity by 49.8% to 72.5%. WithCK and 0.15 mg/L and 0.30 mg/L AZ as a foliar application to 1 d old eggs, intrinsicrates of increase (r_m) of the laboratory population were 0.227, 0.0742 and 0.060,respectively, and net rates of increase (R₀), 36.40, 7.29 and 5.33, respectively; to 5 dold eggs, intrinsic rates of increase (r_m) were 0.126, 0.114 and 0.059, respectively,and net rates of increase (R₀), 39.19, 23.59 and 4.9, respectively.Additionally, effects of foliar and systemically applied AZ on host selection andoviposition of sweetpotato whitefly Bemisia tabaci Gennadius were studied under nochoice, dual choice, and multiple choice conditions. AZ concentrations tersted were1.25, 2.5, 5.0 and 10.0 mg/L, and tap water only was used as control. Under no choiceconditions, the corrected host selection repellency rate reached 82.7% and 48.5% 48 hafter foiiar and systemic treatment of 10 mg/L of AZ, respectively; and 91.0% and61.9% 72h after treatment, respectively. Host selection repellency medianconcentration (RC₅₀) 48h and 72h after folia treatment of AZ were 0.317 mg/L and0.237 mg/L, respectively; and after systemic treatment, 10.301 mg/L and 4.191 rag/L,respectively. Oviposition repellency RC₅₀ 72h after foliar and systemic treatment ofAZ were 1.241 mg/L and 5.833 mg/L, respectively. In multiple choice experiments,selection of cucumber plants by whiteflies negatively correlated with AZconcentrations.The above results show that lethal effect of dipping method is higher thanwrapping application; young nymph is the window stage for whitefly control by AZ;AZ at sublethal concentrations shows significantly inhibitory activities ondevelopment, survival, reproduction and population increase of B. tabaci; and, thereexists strong repellency of both foliar and systemically applied AZ to adult whiteflies,which intensifies with increase of AZ concentration and time period after treatment.Therefore, AZ may be one of the most significant botanical chemicals in the controlof sweetpotato whitefly.