| The control efficiency of imidacloprid to wheat aphids was excellent during the wholegrowing-period. Aiming to investigate the control mechanism thoroughly, in this study, theeffect of wheat aphids’ survival time,honeydew excretion and the sucking ability ofdifferent dosage of Macrosiphum avenae Fabricius on the wheat plant by seed dressingwith imidacloprid at three different stages were studied. The content of imidacloprid inwheat plant and the soil at different times were tested, and evaluated the field controlefficiency. The main results are as follows:Macrosiphum avenae Fabricius’ survival time,honeydew excretion and the suckingability of different dressing dosage of imidacloprid on the wheat plant at three differentstages were tested. It indicated that: At seedling stage, aphids on the seed dressing plant alldied in3h, therefore the honeydew excretion and sucking were obviously lower than CK.At booting stage, the survival rate of wheat aphid on the seed-dressing plant was42%after24h treatment, the number of the sucking was69%compared with CK, the honeydewexcretion decreased by88%~92%compared with CK. Whereas at the seed-filling stage, thesurvival rate of wheat aphid on the seed-dressing plant was77%after24h treatment, thenumber of the sucking was39%compared with CK, the honeydew excretion decreased by80%~91%compared with CK. From the three experiments above, we concluded that, atseedling stage, the aphids were all dead in a very short time after feeding on theseed-dressing plant. It was indicated that imidacloprid had a strong stomach poison activityagainst wheat aphids at this stage. At the booting stage, the survival rate of wheat aphid onthe seed-dressing plant was less than50%after24h treatment. It indicated that it had acertain stomach poison activity against wheat aphids. The number of the sucking was onlydecreased to69%compared with CK, whereas the honeydew excretion was decreasedabove90%. This phenomenon showed that some of the sucking trails were useless or thetime of the feeding in phloem was obviously short because of imidacloprid. It showed anantifeedant effect against wheat aphids. It concluded that the mechanism of imidaclopridagainst aphids was stomach poison cooperated with antifeedant effect at the booting stage.At the seed-filling stage, most of the aphids were survival in the seed-dressing plant after24h treatment, but the feeding of the aphids（honeydew excretion）was still little. It was indicated most of the aphids in the seed-dressing plant at the seed-filling stage were fewdied of direct toxic effect. Most of the aphids stopped or reduced feeding, died of hunger.So it indicated that the seed-dressing plant decreased the feeding of the aphids obviously atthe seed-filling stage, the mechanism of the aphids at this stage was a strong antifeedanteffect.The residues in the wheat plant and the soil were tested, the results showed that: thecontent of imidacloprid in the treated wheat plant was decreased slowly with time passed.The degradation rates were different in the wheat four growing periods. The degradationrate in the wheat plant of the period before winter stage to green stage was higher thanother stages. From green stage to booting stage, the imidacloprid content was trend todecrease. There was not an obvious degradation from the booting to seed-filling stage. Thecontent of imidacloprid was not evenly distributed in wheat plant: the imidacloprid contentof the upper leaves was more than that at the lower part at the booting stage. However, thedifference was disappeared after the seed-filling stage. The imidacloprid content was aboutthe same. The residue in soil was steady falling in the whole growing-season. The highestof the residue in the soli was2.1mg/kg, which was only16%of the maximum levels inwheat plant.The control efficiency of imidacloprid to wheat aphids during the wholegrowing-season was measured in fields by dressing seeds. The results showed that: theduration of imidacloprid against wheat aphids was more than7months only by dressing210~420g/100kg imidacloprid seeds of the recommended dose. The control efficiency ofall dosages was above90%at the period of the earlier aphids happening days. The controlefficiency of dressing seeds with imidacloprid to the mixed aphid population was above90%at the late stage of seed-filling.The control efficiency was different among differentparts of the plant. Residual imidacloprid in wheat kernels which treated by therecommended dose was lower than that specified by national safety standards．... |
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