| The western flower thrips (WFT), Frankliniella occidentalis(Pergande), is a key pest of vegetable, fruit and ornamental crops. It was found first in Beijing in 2003, and then spread throughout Zhejiang, Yunnan, Shandong and Guizhong in China. F. occidentalis not only damages the plants by direct feeding but also transmits many viruses. Currently, control of F. occidentalis relies mainly on chemical insecticides.The present study include:to confirm the best host for rearing F. occidentalis in leguminous vegetables; to screen the insecticides for controlling the F. occidentalis in laboratory; to assess resistance risk for thiamethoxam. This study provides scientific basis and directions for controlling F. occidentalis by insecticides and preventing or slowing the course of resistance to thiamethoxam.1. Confirming the best host for rearing F. occidentalis in leguminous vegetablesTo find a suitable host plant for rearing a sufficient number of F. occidentalis in laboratory, the influence of three selected plants, Semen canavaliae gladiatae, Lablab purpureus(L.) Sweet and Phaseolus vulgaris L, on the development and survival rates of F. occidentalis were studied. Results showed the number of newly-hatched larvae (542.67), survival rates of 2nd to 4th instar (88.67%) and eclosion (71.73%) on Semen canavaliae gladiatae were significantly higher than those on Lablab purpureus(L.) Sweet and Phaseolus vulgaris L.. Therefore the Semen canavaliae gladiatae was the best host for laboratory rearing of F. occidentalis.2. Laboratory screening of insecticides for controlling the F. occidentalisTo provide theoretic basis for the scientific application of insecticides, thirty-eight common insecticides in eight chemical classes were assayed to evaluate their toxicities against F. occidentalis by glass-vial method and glass-vial plus leaf-dipping method in laboratory. The results showed phoxim, butane-fipronil, chlorpyrifos, spinosad, chlorfenapyr, emamectin-benzoate, benfuracarb, thiamethoxam, carbosulfan, acetamiprid, cyhalothrin and bifenthrin could be practical candidates for field use to control F. occidentalis. On the basis, twelve insecticides were assayed to evaluate their toxicities against female adults by glass-vial method in laboratory. Although the order of toxicity was slightly different from that of the larvae's toxicity, the results showed these insecticides were also effective against adult, thus, the above twelve insecticides have potential in controlling F. occidentalis.3. Laboratory screening of insecticide-mixture for controlling the F. occidentalisThe co-toxicity of six insecticides in different ratios was assayed by glass-vial method against the 2nd instar of the F. occidentalis, the results showed nine mixtures have synergism effect. The co-toxicity of butane-fipronil mixed with spinosad in the ratio of 1:3, 1:5,1:7 and 1:9 is 513,348,208 and 591. The ratio of 1:3 and 1:9, which are the best ratio. The co-toxicity of spinosad mixed with emamectin-benzoate in the ratio of 1:1,1:5 is 262, 133. The value of LC50 of the ratio of 1:1 is minimum, so it is the best ratio. The co-toxicity of spinosad mixed with thiamethoxam in the ratio of 1:7,1:9 and 1:11 is 481,368 and 429. The value of LC50 of the ratio of 1:7 is minimum, so it is the best ratio.4. Risk assessment of F. occidentalis to thiamethoxamTo assess resistance risk for thiamethoxam, F. occidentalis was selected with thiamethoxam for 23 generations. The LC50 values increased from 0.004 mg a.i./mL to 0.019 mg a.i./mL, resistant strain was achieved with resistant ratio of 4.75 to thiamethoxam compared with original strain. Based on the method described by Tabashnik, the heritability (h2) of resistance to thiamethoxam in the whole selection stages (F1-F26) was 0.1106. When the selective pressure is 80%-90%, the 10-folds increase in LC50 to thiamethoxam was expected in 12-15 generations, suggesting that F. occidentalis has fairly risk of developing high level resistance to thiamethoxam. |
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