Fitness To Different Soybean Cultivars And Sensitivity To Pesticides Of B-biotype Bemisia Tabaci (Gennadius)

Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) is one of the most important pests in tropical, subtropical regions and adjacent temperate zone. B. tabaci is a highly cryptic group of sibling species composed of a number of biotype or hiding species. The B-biotype of B. tabaci can be easily distinguished from the other kinds of biotypes by the symptoms of silver leaf effect in squash. B-biotype B. tabaci had spread quickly and broke out in many areas since it invaded in early 1990s. Recently B-biotype B. tabaci has becomes serious pest of the vegetables, ornamentals and other economic crops in China. Besides the chemical control measures, application of pest-resistant plant strains turns out to be one of the efficient ways for the control of B. tabaci. In this thesis, laboratory experiments were carried out to study the morphography and the main biological characters of B-biotype B. tabaci on cabbage, to investigate selectivity and fitness to twelve varieties of soybean Glycine max Merrill, senxitivity to pesticides of B-biotype B. tabaci, with the purpose of providing scientific information for application of pest-resistant varieties of soybean, and for the reasonably and efficiently utilization of pesticides against this pest. The results of the experiments were summarized as follows:(1) Our results revealed that the development duration of B-biotype B. tabaci from egg to adult at 26±1℃, photoperiod LD (Light: Dark) 14:10 hours and RH=60% was 18 days on cabbage plant. B-biotype B. tabaci oviposited their eggs on cabbage plant normally in arc or circle, but occasionally in a scatter manner. The body length and the width of both nymph and pupal stages of B-biotype B. tabaci reared on cabbage were much smaller when compared to those reared on cotton and sweet potato.(2) Selectivity and ovipositon preference of B-biotype B. tabaci were significantly different among the twelve tested varieties of soybean. B. tabaci preferred Zhechun No.3, 3618-4, 3659 and Taiwan 75 over Yindou 9701, Chunfengzao, Xiaoken 8901, Ribenaijiaozao, and it had least preference for Liaoxian No.1, 303, 2818, 5702. There were significant differences in their development duration and survival rate among the twelve varieties of the soybean. The development duration of B-biotype B. tabaci from egg to adult is longest (20.41 days) on the 303 whereas it was shortest on the Chunfengzao (18.43 days). No significant difference had been found in the survival rate for the second nymph instar among all tested soybean cultivars although the survival rate in the other stages had significant differences.(3) B. tabaci had the most fitness, rapid population increase, longest adult longevity and higher female fecundity on 3618-4, Zhechun No.3 and Xiaoken 8901 whereas it had the poorest fitness, the lowest population increase, the shortest adult longevity and the least fecundity on 5702 and Liaoxian No.l, suggesting that 5702 and Liaoxian No.1 can be used as pest-resistant cultivars.(4) The toxicity of nine insecticides on the adult, nymph and egg stages were evaluated usingthe leaf dip method under laboratory conditions. Buprofezin caused the lowest mortality (6.95%) of B. tabaci adults within 24 hours, which did not show different to that in control. Among the other eight insecticides, the toxicity of abamectin and acetamiprid is the highest, with the LC₅₀ of 0.01 and 2.85 mg/L, respectively. The toxicity of rotenone was the lowest, with the LC₅₀ of 6826.92 mg/L. Generally speaking, the toxicity order of tested insecticides against the adult stage was as follows abamectin > acetamiprid >azadirachtin>oxymatrine>imidacloprid> chlorpyrifos > cyhalothrin > rotenone. The toxicity of the tested insecticides to adults was increased with the extension of the exposure time, and abamectin could produce 100% mortality of the adults within 48 hours.As against the youngest (first and second instars) nymph, abamectin, acetamiprid and oxymatrine had the highest toxicity, with the LC₅₀ of 0.19, 1.28 and 2.81 mg/L, respectively, while cyhalothrin and chlorpyrifos had the lowest toxicity, with the LC₅₀ of 419.21 and 626.13 mg/L, respectively. The toxicity order of nine pesticides to youngest nymph was: abamectin> acetamiprid > oxymatrine > azadirachtin > imidacloprid > rotenone > buprofezin > cyhalothrin > chlorpyrifos. When compared their toxicity against the elder nymph (third and fourth instars), again, abamectin, oxymatrine and acetamiprid showed the highest toxicity, with the LC₅₀ of 0.47, 0.90 and 1.33 mg/L, respectively. Cyhalothrin and chlorpyrifos showed the lowest toxicity, with the LC₅₀ of 899.16 and 1286.56 mg/L, respectively. Their toxicity order to the third and fourth nymphal instar was as follows: abamectin > oxymatrine > acetamiprid > azadirachtin > imidacloprid>rotenone>buprofezin>chlorpyrifos>cyhalothrin. No toxicity to the pest eggs had been found in all tested insecticides. (5) The adults of B-biotype B. tabaci, which had exposed to azadirachtin coated on cabbage leaves for 2 days, laid significant fewer eggs than those that had not exposed to azadirachtin, and the eggs laid by the former gave significant higher hatching percentage than those laid by the latter. Meanwhile, these differences increased with the increasing concentration of azadirachtin. However, these differences decreased with the increasing duration after exposure. The eggs laid by adults, which were first exposed to cabbage plant treated with 40 mg/L azadirachtin for 2 days and then transferred to fresh cabbage plant with no azadirachtin pollution for 6 days, produced similar hatching percentage to those eggs laid by the adults which never had exposed to azadirachtin on the sixth day. These results indicated that azadirachtin could penetrate the body wall of B-biotype B. tabaci, and then affect the pest fecundity and the hatching percentage of resultant eggs. The experimental results of penetrating capacity of four insecticides indicated that oxymatrine had the highest penetration, azadirachtin and abamectin followed afterward, rotenone fell the last when applied in the tested concentrations.