Abstract: The relationship between wheat aphids field efficacy and imidacloprid concentration,droplet deposition density was studied.Field application of imdacloprid 700WG with knapsack mist-blower showed that certain droplets deposition density on wheat head can effectively control wheat aphids.Seven days after imidacloprid spraying,the field efficacy against wheat aphids were 83.3%,88.7% and 93.7% when droplet deposition density were 54,133 and 280 particle/cm2,respectively at concentration of 300 mg/L;and the field efficacy were 88.1%,94.5% and 96.5% when droplets deposition density were 75,142 and 291 particle/cm2,respectively at concentration of 600 mg/L.When field spraying with knapsack mist-blower at volume of 150 L/ha and 75 L/ha,the droplet deposit density on wheat head,flag leaf were 177±12,113±8 and 127±13,91±7 particle/cm2,respectively;and the run-off rates of spray on field ground surface were 12.8% and 10.6%,respectively.There was no significant difference of spray deposit dosage on wheat head,flag leaf between spraying with low volume of 75 L/ha at 600 mg/L and spraying with volume of 150 L/ha at 300 mg/L;and the field efficacy against wheat aphids were 94.0% and 93.8%,respectively.When applying imdidacloprid against wheat aphids with knapsack mist-blower,low volume of 75 L/ha at high concentration of 600 mg/L was preferred for labor,water and time saving.